Calcium dependence of ouabain-induced contraction in aortas from spontaneously hypertensive rats

Abstract The calcium sensitivity of ouabain-induced contractions of aortic strips from spontaneously hypertensive rat (SHR) was examined using several drugs which affect Na + and Ca 2+ movements across the cell membrane, and the results were compared with those obtained with age-matched Wistar-Kyoto rat (WKY). The Ca 2+ concentration-response curves (10 −3 M ouabain-treated preparations) made with aortic strips from SHR lay to the left of those made with aortic strips from WKY (Ca EC 50 values: SHR, 0.51 ± 0.16 mM, n = 6; WKY, 1.23 ±0.41 mM, n = 7; P + entry blocker) and nifedipine (a Ca 2+ entry blocker) attenuated the sensitivity to Ca 2+ of SHR and WKY aortic strips. With 2 × 10 −4 M amiloride, WKY vessels showed a 1.3-fold increase in the Ca EC 50 value and SHR a 2.1-fold increase. With 10 −6 M nifedipine. WKY vessels showed a 1.1-fold increase in the Ca EC 50 value and SHR a 1.5-fold increase. Addition of monensin (Na ionophore) produced a dose-dependent potentiation in ouabain-treated aorta from WKY, but not in ouabain-treated aorta from SHR. Addition of 1.5 × 10 −5 M A23187 (Ca ionophore) eliminated the difference between the Ca 2+ -induced contractions in aortas from SHR and WKY. These results suggest that enhancement of Ca 2+ influx by Na + -Ca 2+ exchange and/or voltage-dependent Ca 2+ channels in vascular smooth muscle cell membranes may be an important factor in the difference between ouabain-induced contractions in aorta from SHR and WKY.