For 50 years after its discovery in 1926, there was a general lack of interest in relaxin among both reproductive biologists and clinicians. A key reason for this lack of interest was the lack of information concerning relaxin's physiological importance during pregnancy in any species. Research conducted since the early 1980s has established that the hormone relaxin is essential during pregnancy in at least two species--rats and pigs. Two vital roles for relaxin during pregnancy have been identified. Relaxin promotes growth and softening of the uterine cervix and thereby enables rapid and safe delivery in both rats and pigs. Relaxin also promotes growth and development of the mammary apparatus in both species. Interestingly, the major effects of relaxin on mammary growth and development are targeted on the nipple in the rat, whereas they are targeted on the glandular parenchyma in the pig. Relaxin-dependent growth of the nipple in rats is required for normal lactational performance. Although likely, it remains to be established that relaxin's profound effects upon mammary gland development in pigs are required for normal lactational performance. The fact that relaxin has effects upon cervical and mammary gland development during pregnancy in both rats and pigs encourages the view that relaxin may have similar effects during pregnancy in other species. Nevertheless, one must keep in mind that there is great diversity in the physiology of relaxin among species (reviewed by Sherwood 1988). This diversity includes not only relaxin's source, regulation of synthesis and secretion, and secretory profiles during pregnancy, but also its biological effects. It seems essentially certain that relaxin's effects during pregnancy differ among species. For example, transformation of the pubic joint cartilage to a flexible and elastic interpubic ligament occurs during pregnancy in several species, including guinea pigs, mice, and bats. This pelvic adaptation, which is nearly certainly relaxin dependent, does not occur in species such as rats and sheep. It is possible that relaxin may have little or no physiological significance during pregnancy in some species. Although considerable progress has been made toward an understanding of the physiological role(s) of relaxin in pregnant rats and pigs, many fundamental questions remain unanswered.(ABSTRACT TRUNCATED AT 400 WORDS)
Cervices from day 18 (D18) intact pregnant rats show significantly greater extensibility and ability to accommodate to extension than cervices from D9 pregnant rats. On D22, cervices from intact pregnant rats show even greater extensibility and accommodation to stretch. Cervices from ovariet mized pregnant rats treated with estrogen (E) and progesterone (P; group OPE) show markedly reduced extensibility and ability to accommodate to stretch compared with intact pregnant cervical tissue (group C) on both D18 and D22. Extensibility of group OPE cervices resembles that of cervices from D9 intact pregnant rats. Cervices from ovariectomized pregnant rats treated with E, porcine relaxin (R), and P (group OPER) or porcine R and P (group OPR) exhibit similar extensibility and ability to accommodate to stretch as cervices from intact pregnant rats on both D18 and D22. The importance of R for cervical softening during pregnancy and its interaction with E near term and during parturition are discussed. (Endocrinology116: 1215–1220, 1985)
1 The kinetics of diltiazem were investigated in ovariectomized (ovx) non-pregnant and intact late pregnant anaesthetized rats following a bolus i.v. injection (2 mg kg−1) and during a 180 min i.v. infusion (50 μg kg−1 min−1 and 100 μg kg−1 min−1). Uterine contractions, mean blood pressure and heart rate were measured in the non-pregnant rats. 2 Measurement of serum diltiazem concentrations after bolus i.v. injection in ovx non-pregnant rats showed a biexponential decay with time from which the following parameters were calculated: volume of distribution area (V(area)) − 256 ±46 ml; rate constants k12 − 0.46 ± 0.10 min−1; k21 − 0.09 ± 0.01 min−1; kel − 0.13 ± 0.03 min−1; elimination clearance − 3.2 ± 0.3 ml min−1; distribution t1/2 (t1/2α) − 1.4 ± 0.3 min; elimination t1/2 (t1/2β) − 61.2 ± 13.0 min. In pregnant rats, a biexponential decay was also observed with similar parameters to those in non-pregnant animals except for markedly increased V(area) − 1004 ± 184 ml; kel − 0.54 ± 0.16 min−1 and elimination clearance − 14.8 ± 2.3 ml min−1. 3 Measurement of serum diltiazem concentrations during infusion yielded the following parameters in non-pregnant ovx rats: V(ss) − 79 ± 10 ml; rate constants k12 − 1.02 ± 0.21 min−1; k21 − 0.03 ± 0.01 min−1; kel-0.39 ± 0.06 min−1; elimination clearance-7.8 ± 1.2 ml min−1. In pregnant rats a marked increase was observed in kel − 1.25 ± 0.38 min−1 and elimination clearance − 36.4 ± 13.8 ml min−1. 4 An immediate reduction in uterine contractions, mean blood pressure and heart rate was observed after bolus i.v. injection of diltiazem with a return towards control values as serum diltiazem concentrations declined. There were significant correlations between the inhibition of the 3 parameters and the log serum concentrations of diltiazem. Serum concentration-response curves indicated IC50 values of 0.5 μg ml−1 for inhibition of uterine contractions, 0.7 μg ml−1 for reduction in blood pressure and 1.2 μg ml−1 for reduction in heart rate. There were maintained reductions in the integral of uterine contractions, mean blood pressure and heart rate during infusion. 5 The metabolite desacetyldiltiazem was rarely detected after i.v. bolus injection and was not found in 5/13 rats infused with diltiazem, yet significant inhibition of uterine contractions was observed in all rats. Diltiazem was 3.2 fold more potent than desacetyldiltiazem as an inhibitor of contractions of the rat isolated uterus. 6 These findings indicate that the inhibition of uterine contractions is due to a direct action of diltiazem, and not the metabolite desacetyldiltiazem, and suggest only a slight selectivity for uterine inhibition compared to cardiovascular effects.
Platelet-activating factor (PAF) is produced by preimplantation embryos and may be involved in the earliest stages of embryo-maternal dialogue. This study explored the potential effects of PAF acting as a signalling agent on human Fallopian tubal epithelial cells grown as a polarized layer in primary culture. The response of the tubal epithelium was assessed in terms of the transepithelial potential difference and short-circuit current (I(scc)), which were recorded using a modified Ussing chamber. Resistance was calculated from the measurements of potential difference and I(scc). PAF (1.9 nmol to 1.9 micromol l(-1)) administered to the apical surface of the cells produced a marked, transient increase in both potential difference and I(scc) in a dose-dependent manner. The mode of action of PAF on the electrophysiological responses of human tubal epithelial cells was investigated. Blockers of Na(+), K(+) and voltage-operated Ca(2+) channels had little effect on PAF action. However, incubation of the epithelial cells in Cl(-)free medium or with a blocker of the Na(+)-K(+)-2Cl(-) cotransporter (Furosemide) reduced the effect of PAF. Blockade of chloride-bicarbonate channels with 4-acetamido-4'-iso-thiocyanostilbene-2.2'-disulphonic acid (SITS) reduced the effect of low doses of PAF only. These results indicate that PAF influences the movement of chloride ions across the tubal epithelial cell and is a candidate molecule for initial embryo-maternal dialogue.
The kinetics of nifedipine and the relationship between its serum concentration and uterine and cardiovascular effects were investigated in 3 groups of animals. These were ovariectomized (ovx) anaesthetized non‐pregnant rats following bolus i.v. injection (400 μg kg −1 ) and during 300 min infusion (10 μg kg −1 min −1 ) and ovx, progesterone‐treated late pregnant rats during infusion. Also, the kinetics were determined in ovary‐intact late pregnant rats following bolus i.v. injection (400 μg kg −1 ). Measurement of serum nifedipine concentrations after bolus i.v. injection in ovx non‐pregnant rats showed a biexponential decay with time from which the following parameters were calculated: V β = 300 ± 30 ml kg −1 ; rate constants k 12 = 0.51 ± 0.18 min −1 ; k 21 = 0.07 ± 0.02 min −1 ; k el = 0.10 ± 0.05 min −1 ; elimination clearance = 2.4 ± 0.2 (ml min −1 )kg −1 ; t 1/2α = 2.5 ± 1.0 min; t 1/2β = 102 ± 15 min. In intact pregnant rats, a biexponential decay of serum nifedipine concentrations with time was also observed after bolus i.v. administration with similar parameters to non‐pregnant animals. These kinetic parameters, used to calculate serum nifedipine concentrations obtained during infusion, predicted values similar to experimental values for 180 min, but thereafter slightly underestimated experimental values. Immediate reductions in uterine contractions, mean blood pressure and heart rate were observed following bolus i.v. injection of nifedipine to ovx non‐pregnant rats, with returns towards control values as serum nifedipine concentrations declined. IC 15 values (15% change from baseline), calculated from log 10 serum concentration‐response curves, of 0.3 ± 0.05 μg ml −1 for inhibition of uterine contractions, 0.8 ± 0.3 μg ml −1 for depression of blood pressure and 3.8 ± 1.0 μg ml −1 for reduction in heart rate were obtained. In ovx non‐pregnant rats, nifedipine infusion produced a maximum reduction in integral of uterine contractions of 70% by 120 min and a maximum reduction of 15% in heart rate. Mean blood pressure was not significantly different from vehicle‐treated rats. IC 15 values were 0.7 ± 0.1 μg ml −1 and 2.8 ± 0.6 μg ml −1 for inhibition of uterine contractions and heart rate respectively. In ovx, progesterone‐treated late pregnant rats, nifedipine infusion produced similar serum concentrations to those of non‐pregnant rats but completely abolished uterine contractions by 70 min. Maximum reductions of 30% in heart rate and blood pressure were observed. IC 15 values were 0.5 ± 0.1 μg ml −1 for uterine contractions, 0.9 ± 0.3 μg ml −1 for blood pressure and 1.2 ± 0.3 μg ml −1 for heart rate. The findings suggest that the kinetics of nifedipine are similar in pregnant and non‐pregnant rats and support the idea that the drug exerts a slight selectivity for uterine inhibition relative to cardiovascular effects. The uterus of the late pregnant rat appears to be more sensitive to nifedipine than that of the non‐pregnant animal.
Porcine relaxin (250 guinea-pig units/mg) infused intravenously into anaesthetized rats at 20 micrograms/h reversibly abolished spontaneous intra-uterine pressure cycles yet left the myometrium responsive to oxytocin in doses of 4--8 mu. The inhibition was found to be primarily of the frequency, rather than of the amplitude, of pressure cycles. Relaxin (5 or 10 micrograms) was capable of completely suppressing uterine activity driven by prostaglandin F2 alpha infusion in oestrogen-treated ovariectomized rats. Whereas the beta-adrenergic blocker, propranolol, had no effect on relaxin-induced inhibition, phentolamine, an alpha-blocker, significantly delayed the relaxin effect. It is unlikely, however, that relaxin operates through an alpha-inhibitory receptor. The results show that relaxin acts primarily as a frequency modulator and is capable of antagonizing an exogenous myometrial stimulant.
Ovariectomized post-partum rats exhibit in vivo continuous stable myometrial activity with a frequency of 45--50 pressure cycles per h, and a mean maximum amplitude of 45--50 mmHg for many days. Oestradiol benzoate (5 micrograms) reduced the frequency of intra-uterine pressure cycles to 5 cycles per h by 20 h after treatment. The decrease in frequency was due to increased periods of uterine quiescence. Pretreatment with reserpine caused significant reductions in the concentration of uterine adrenaline and noradrenaline as measured by a fluorometric assay but had no effect on the extent or time-course of oestrogen-induced myometrial quiescence. Neither intravenous infusion of the adrenergic beta-blocker, propranolol, altered the extent or the time-course of the reduction of intra-uterine pressure cycles after oestrogen treatment. These results suggest that the mechanism by which oestrogen induces myometrial quiescence does not involve adrenaline mediation or alpha- or beta-adrenoceptor activation.
Relaxin and progesterone are produced by the corpora lutea in the pregnant rat. Relaxin immunoactivity levels are elevated in peripheral sera during the last 12 days of pregnancy. In rats maintained under a conventional photoperiod of 14 h of light (0600–2000 h) and 10 h of darkness (2000–0600 h), there is an antepartum elevation in serum relaxin to maximal levels coincident with the rapid decline in serum progesterone to basal levels during the light phase of the photoperiod 1 day before birth. Therefore, we postulated that this maximal elevation in serum relaxin levels may be temporally associated with functional luteolysis and linked to the photoperiod. In the present study the photoperiod was advanced near midpregnancy in order to examine further the relationship of the antepartum elevation in serum relaxin levels with both functional luteolysis and the photoperiod. Three groups of rats were maintained under a conventional photoperiod of 14 h of light (0500-1900 h) and 10 h of darkness until days 7 and 8 of pregnancy when the photoperiod was advanced 8 h in group 2 (G2) and advanced 18 h in G3 relative to the conventional photoperiod that was maintained in Gl. Serum relaxin and progesterone levels were determined in blood samples obtained at 4-h intervals from 2000 h on day 19 of pregnancy until birth. The times of occurrence of birth, maximal relaxin levels, and decline of progesterone to basal levels in G2 and G3 were generally advanced 50–60% of the advancement of the photoperiod. There was a close temporal association between the attainment of maximal relaxin levels and basal progesterone levels; they both occurred during the light phase of the photoperiod, approximately 24 h before birth in all three groups. We conclude that the antepartum elevation of serum relaxin to maximal levels may be associated with functional luteolysis and that its time of occurrence is influenced by the photoperiod. This study also provides evidence that the antepartum elevation of relaxin levels consists of two phases which occur at a 24-h interval. It is proposed that these two phases in the elevation of relaxin levels may be indicative of an increasingly effective endogenous circadian luteolytic process whose time of occurrence is influenced by the light-dark schedule. (Endocrinology113:997, 1983)
ABSTRACT The influence of oestradiol benzoate and progesterone on uterine sensitivity and development of tolerance to relaxin was investigated in bilaterally ovariectomized non-pregnant rats in vivo . Bolus doses of relaxin (2–20 μg/kg i.v.) produced rapid and reversible inhibition of uterine contractions in a dose-dependent manner. Treatment with oestradiol benzoate or oestradiol benzoate plus progesterone significantly increased uterine sensitivity to relaxin over 48 h by 2·4- to 8·5-fold. Tolerance to relaxin developed during continuous infusion of the hormone at 20 μg/kg per h for 40 h. A 7·8- to 17·4-fold reduction in sensitivity to relaxin was observed in relaxin-infused rats, whereas no change in sensitivity was observed in saline-infused rats. Infusion of relaxin at 50 μg/kg per h for 40 h produced a 131·8-fold reduction in uterine sensitivity to relaxin. The uterus remained tolerant to relaxin for up to 24 h after cessation of infusion. Treatment with oestradiol benzoate and/or progesterone did not influence the extent of tolerance development, but a more rapid recovery of uterine sensitivity to relaxin was observed in rats treated with oestradiol benzoate plus progesterone. Cross-tolerance with other uterine relaxant drugs was measured to investigate possible common mechanisms of action and sites of tolerance between relaxin and a β-adrenoceptor agonist (salbutamol) and potassium channel openers (cromakalim and minoxidil sulphate). No cross-tolerance was observed between relaxin and salbutamol, or relaxin and cromakalim or minoxidil sulphate. Cross-tolerance between cromakalim and minoxidil sulphate was seen. Journal of Endocrinology (1992) 135, 17–28
Studies of mechanical activity and 86 Rb + efflux have been made in bovine isolated trachealis with the objectives of (a) identifying which of the β‐adrenoceptor subtypes mediates the opening of plasmalemmal K + ‐channels, (b) gaining further insight into the properties of the novel, long‐acting β 2 ‐adrenoceptor agonist, salmeterol and (c) clarifying the role of K + ‐channel opening in mediating the mechano‐inhibitory actions of agonists at β‐adrenoceptors. In bovine trachealis muscle strips precontracted with histamine (460 μ m ), isoprenaline (0.1 n m – 1 μ m ), procaterol (0.1–10 n m ) and salmeterol (0.1–10 n m ) each caused concentration‐dependent relaxation. ICI 118551 (10 n m –1 μ m ) antagonized isoprenaline, procaterol and salmeterol in suppressing histamine‐induced tone of the isolated trachealis muscle. The antagonism was concentration‐dependent. In contrast, CGP 20712A (10 n m –1 μ m ) failed to antagonize isoprenaline, procaterol or salmeterol. Salmeterol (1–10 μ m ) antagonized isoprenaline in relaxing strips of bovine trachea which had been precontracted with carbachol (1 μ m ). Cromakalim (10 μ m ), isoprenaline (100 n m –10 μ m ), procaterol (10 n m –1 μ m ) and salbutamol (100 n m –10 μ m ) each promoted the efflux of 86 Rb + from strips of bovine trachealis muscle preloaded with the radiotracer. In contrast, salmeterol (100 n m –10 μ m ) failed to promote 86 Rb + efflux. CGP 201712A (1 μ m ), ICI 118551 (100 n m ) and salmeterol (1 μ m ) did not themselves modify 86 Rb + efflux from trachealis muscle strips, nor did they affect the promotion of 86 Rb + efflux induced by cromakalim (10 μ m ). In contrast, CGP 20712A (1 μ m ) and ICI 118551 (100 n m ) were each able to inhibit the promotion of 86 Rb + efflux induced by isoprenaline (1 μ m ) or procaterol (100 n m ). Furthermore, salmeterol (10 μ m ) inhibited isoprenaline (1 μ m )‐induced promotion of 86 Rb + efflux. It is concluded that, in bovine trachealis, activation of either β 1 ‐ or β 2 ‐adrenoceptors can promote the opening of 86 Rb + ‐permeable K + ‐channels in the plasmalemma. The failure of salmeterol to promote plasmalemmal K + ‐channel opening may reflect, not its selectivity in activating β 2 ‐ as opposed to β 1 ‐adrenoceptors, but rather its low intrinsic efficacy at β 2 ‐adrenoceptors. The opening of plasmalemmal K + ‐channels plays a supportive rather than a crucial role in mediating the mechano‐inhibitory effects of agonists at β‐adrenoceptors acting on trachealis muscle.
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