DEPOLARIZATION PROMOTES CAFFEINE INDUCED [H-3] NORADRENALINE RELEASE IN CALCIUM-FREE SOLUTION FROM PERIPHERAL SYMPATHETIC-NERVES

The transmitter releasing action of caffeine was studied in the absence of extracellular Ca2+ from the peripheral sympathetic nerves of the rabbit main pulmonary artery. Caffeine (10 mM) increased the release of [3H]-noradrenaline moderately, but not significantly in Ca2+-free (+1 mM EGTA) Krebs solution. When peripheral nerve endings/varicoslties were depolarized by elevating extracellular K+ to 47.2 mM and 70.8 mM in Ca2+-free solution, the transmitter releasing effect of 10 mM caffeine became significant. Ca2+ removal itself transiently increased the [3H]-noradrenaline outflow. In the individual experiments the amount of the caffeine evoked transmitter release at 47.2 mM and 70.8 mM K+-depolarization was inversely correlated to the release evoked by Ca2+-removal. Our results suggest that caffeine-sensitive calcium stores are present in peripheral nerve terminals of rabbit pulmonary artery, and part of the caffeine sensitive calcium stores may discharge during Ca2+-removal from the extracellular solution. © 1990.