INCREASES IN [CA2-RAT SYMPATHETIC NEURONS ARE NOT DEPENDENT ON INTRACELLULAR CA2+ POOLS(](I) BY CCH IN ADULT)

We have examined the effects of the muscarinic agonists, carbachol (CC h) and oxotremorine (Ore), on the intracellular free Ca2+ concentratio n ([Ca2+](i)) in acutely dissociated sympathetic neurons from adult ra ts using fura 2-based microfluorometry. The drugs increased [Ca2+](i) by 86 +/- 7 and 38 +/- 10 nM for CCh and Oxo, respectively (both 10 mu M). Basal [Ca2+](i) was 52 +/- 3 nM. Depletion of the caffeine-sensit ive Ca2+ store or blockade of the Ca2+-adenosinetriphosphatase with th apsigargin did not alter the effect of either agonist on the rise in [ Ca2+](i). On the other hand, the omission of Ca2+ from the perfusion s olution or the use of TA-3090, a Ca2+ channel antagonist, blocked the effects of CCh and Ore. In whole cell current-clamp recordings, the mu scarinic agonists elicited a depolarization and action potential firin g, which probably explained the rise in [Ca2+](i) observed with microf luorimetric recording. In addition to their direct effects on the [Ca2 +](i), muscarinic agonists also reduced the rise in [Ca2+](i) induced by a nicotinic agonist. This inhibitory effect, observed in 68% of cel ls that responded to the nicotinic agonist, was blocked by atropine an d pertussis toxin, whereas the muscarinic agonist-induced increase in [Ca2+](i) was blocked by atropine but was pertussis toxin insensitive. These results suggest that at least two muscarinic receptors are pres ent on sympathetic neurons and that they mediate opposite effect on th e fluctuation of [Ca2+](i).