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) ⁴⁵Ca²⁺ influx in ring preparations, and (3) Ca²⁺-sensitivity of the contractile apparatus in α-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 ⁴⁵Ca²⁺ 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 ⁴⁵Ca²⁺ 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.