Release of Ca2+ by noradrenaline and ATP from the same Ca2+ store sensitive to both InsP3 and Ca2+ in rat portal vein myocytes.

1. Changes in cytosolic free Ca2+ concentration ([Ca2+]i) induced by noradrenaline (NA) and ATP were investigated using indo‐1 microspectrofluorimetry in single smooth muscle cells of rat portal vein. 2. Activation of P2x‐purinoceptors by ATP (10 microM) increased [Ca2+]i from 92 +/‐ 7 nM (n = 18) to 557 +/‐ 30 nM (n = 11). In the presence of NA (10 microM), the ATP‐induced rise in [Ca2+]i was reduced to 23.6 +/‐ 1.5% (n = 7) of the control response (in the absence of NA). 3. Tetracaine (10 microM to 2 mM) inhibited in a concentration‐dependent manner the Ca(2+)‐induced Ca2+ release (CICR) evoked by 5 mM caffeine. In the presence of 1 mM tetracaine, the rise in [Ca2+]i induced by ATP (10(‐8)‐10(‐4) M) was strongly inhibited. A tetracaine‐resistant rise in [Ca2+]i, corresponding to 26.4 +/‐ 2.3% (n = 14) of control values, was recorded in response to 10 microM ATP. 4. The amplitude of the NA‐induced [Ca2+]i rise depended on NA concentrations (10(‐8)‐10(‐5) M) and was not modified by tetracaine (1 mM). 5. This study suggests that Ca2+ ions released through the InsP3 receptor‐channel upon NA application do not activate CICR and the InsP3‐ and Ca(2+)‐sensitive Ca2+ store appears to represent, at least functionally, a single releasable Ca2+ pool.