CALCIUM DEPENDENCE OF OUABAIN-INDUCED CONTRACTION IN AORTAS FROM SPONTANEOUSLY HYPERTENSIVE RATS

The calcium sensitivity of ouabain-induced contractions of aortic strips from spontaneously hypertensive rat (SHR) was examined using several drugs which affect Na+ and Ca2+ movements across the cell membrane, and the results were compared with those obtained with age-matched Wistar-Kyoto rat (WKY). The Ca2+ 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 EC50 values: SHR, 0.51 ± 0.16 mM, n = 6; WKY, 1.23 ±0.41 mM, n = 7; P < 0.05). Amiloride (a Na+ entry blocker) and nifedipine (a Ca2+ entry blocker) attenuated the sensitivity to Ca2+ of SHR and WKY aortic strips. With 2 × 10-4 M amiloride, WKY vessels showed a 1.3-fold increase in the Ca EC50 value and SHR a 2.1-fold increase. With 10-6 M nifedipine. WKY vessels showed a 1.1-fold increase in the Ca EC50 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 Ca2+-induced contractions in aortas from SHR and WKY. These results suggest that enhancement of Ca2+ influx by Na+-Ca2+ exchange and/or voltage-dependent Ca2+ 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. © 1990.