Hm. Cousins et Nj. Bramich, EFFECTS OF SYMPATHETIC-NERVE STIMULATION ON MEMBRANE-POTENTIAL, [CA2-MARINUS(](I) AND FORCE IN THE ARRESTED SINUS VENOSUS OF THE TOAD, BUFO), Journal of physiology, 505(2), 1997, pp. 513-527
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.