Endogenous Cardiotonic Steroids and Differential Patterns of Sodium Pump Inhibition in NaCl-Loaded Salt-Sensitive and Normotensive Rats

Salt sensitivity contributes to elevated blood pressure (BP) in 30–40% of hypertensives worldwide.1,2 Endogenous digitalis-like cardiotonic steroids (CTS) are stimulated by renal sodium retention and by plasma volume expansion and are implicated in the homeostatic response to NaCl loading.3 In normo-tensives, CTS, acting via inhibition of renal sodium pump, promote sodium excretion.4,5 In salt-sensitive subjects, the impaired renal sodium pump is one of the factors underlying renal sodium retention.4,5 In these subjects, the excessive elaboration of CTS occurs with an adaptive effect, to override sodium retention. However, heightened CTS levels also exhibit a maladaptive effect, inhibit Na,K-ATPase (NKA) in vascular sarcolemma and raise BP.3–5 α-1 NKA in vitro and exhibits pressor and natriuretic effects in vivo, has been identified in human and rat tissues.7–11 Genetically determined hypertension that develops in Dahl salt-sensitive rats (DS) on a high NaCl diet results from renal sodium retention and, at least in part, is mediated by increased elaboration of MBG from adrenal cortex.12 In previous studies of mechanisms of salt sensitivity of BP, DS were usually compared to their salt-resistance counterparts, Dahl salt-resistant rats (DR).9,10,12 In the present study, we compared DS to Sprague-Dawley rats (S-D) because S-D, a genetic precursor of DS, at least with respect to CTS responses, may be more relevant to the normotensive human population than DR, in which salt-resistance has been genetically reinforced. 12 While in NaCl-loaded DS, markedly elevated MBG levels mediate NaCl-induced hypertension,13 in DR following acute and especially chronic NaCl loading, levels of MBG exhibit minimal changes.10,13 Moreover, renal NKA from DS, unlike that from other strains of rats, is remarkably insensitive to MBG.10 In NaCl-loaded S-D, however, a fourfold increase in renal MBG excretion does not increase BP but mediates a natriuretic response, as in these rats, the anti-MBG antibody administration in vivo markedly reduces renal sodium excretion and restores renal NKA activity.14 Likewise, in the normotensive NaCl-loaded human subjects, elevated levels of MBG correlate with renal sodium excretion in the presence of modestly elevated systolic BP (SBP).15 We hypothesize that in NaCl-loaded DS and in normotensive S-D, increased MBG levels would elicit different patterns of NKA inhibition and cause preferential inhibition of the sodium pump in vascular sarcolemma in DS and in kidney of S-D. Recently, we have demonstrated that atrial natriuretic peptide (ANP), via cyclic guanosine monophosphate (cGMP)-dependent NKA phosphorylation, potentiates the effect of MBG on renal sodium pump but exhibits an opposite effect in the vasculature.16 We, therefore, further hypothesized that impaired ANP response may be one of the factors underlying differential responses of the sodium pump to NaCl loading in S-D and DS. In the present experiment, we compared effects of NaCl loading on levels of endogenous ouabain (EO), MBG, ANP, and cGMP and the activity of sodium pump in thoracic aorta and renal outer medulla in DS and S-D.