Changes in the cytosolic Ca2+ concentration and Ca2+-sensitivity of the contractile apparatus during angiotensin II-induced desensitization in the rabbit femoral artery

1 To investigate the underlying mechanism for the angiotensin II-induced desensitization of the contractile response during the prolonged stimulation of the Vascular smooth muscle, we determined the effects of angiotensin-II on (1) cytosolic Ca2+ concentration ([Ca2+](i)) and tension using fura-2-loaded medial strips of the rabbit femoral artery, (2) Ca-45(2+) influx in ring preparations, and (3) Ca2+-sensitivity of the contractile apparatus in alpha-toxin permeabilized preparations. 2 In the presence of extracellular Ca2+, high concentrations of angiotensin-II elicited biphasic increases in [Ca2+](i) and tension, which consisted of initial transient and subsequent lower and sustained phases. 3 The Ca-45(2+) influx initially increased after the application of 10(-6) M angiotensin-II, and thereafter gradually decreased. At 20 min after the application, there was a discrepancy between the level of [Ca2+](i) and the extent of Ca-45(2+) influx. 4 The relationships between [Ca2+](i) and tension suggested that the angiotensin-II-induced increase in the Ca2+-sensitivity of the contractile apparatus was maintained during the desensitization of smooth muscle contraction. 5 When 10(-6) M angiotensin-II was applied during the sustained phase of contraction induced by 118 mm K+-depolarization, at 10 min after the application, the [Ca2+](i) levels were significantly lower and the tension levels were significantly higher than those prior to the application of angiotensin-II. 6 In conclusion, the decrease in [Ca2+](i), which is partially due to the inhibition of the Ca2+ influx, is mainly responsible for the desensitization evoked by high concentrations of angiotensin-II, and angiotensin-II seems to activate additional mechanisms which inhibit Ca2+ signaling during prolonged stimulation.