EFFECTS OF SYMPATHETIC-NERVE STIMULATION ON MEMBRANE-POTENTIAL, [CA2-MARINUS(](I) AND FORCE IN THE ARRESTED SINUS VENOSUS OF THE TOAD, BUFO)

1. The effects of sympathetic nerve stimulation on membrane potential and on the intracellular concentration of calcium ions, [Ca2+](i), wer e recorded concurrently from the sinus venosus of the toad, Bufo marin us, in preparations where beating had been abolished by adding an orga nic calcium antagonist to the physiological saline. In a separate set of experiments the effects of sympathetic nerve stimulation on force p roduction were examined. 2. Stimulation of the sympathetic nerves caus ed a membrane depolarization and a simultaneous increase in [Ca2+](i). Both responses were reduced by dihydroergotamine (20 mu M). 3. The me mbrane depolarization and increase in [Ca2+](i) evoked by sympathetic nerve stimulation were abolished by ryanodine (10 mu M), or caffeine ( 3 mM). The effects of caffeine, but not those of ryanodine, were fully reversible. 4. Although the Ca2+-ATPase inhibitor thapsigargin (30 mu M) itself had little effect on the responses to sympathetic nerve sti mulation, in its presence caffeine (3 mM) irreversibly abolished the r esponses. 5. In the presence of nifedipine (10 mu M), sympathetic nerv e stimulation caused contractions of the sinus venosus. These response s were abolished by either ryanodine (10 mu M) or caffeine (3 mM). 6. The results suggest that neuronally released transmitter activates a c omplex biochemical pathway which triggers the release of Ca2+ from int ernal stores.