Effects of Intracellular Ca2+ on the Frog Muscle Spindle in Relation to Cyclic AMP Action
In sensory systems, cyclic nucleotides or calcium ions (Ca2+) have been proposed as intracellular messengers for coupling the light activation of the photopigment rhodopsin to channel activity and thus modulating light-sensitive conductance (Lipton et al. 1977; Miller and Nicol, 1979; Fesenko et al., 1985), or for coupling between the odorant effect and olfactory transduction (Pace et al., 1985). Recently we have found that application of dibutyryl cyclic AMP, forskoline (adenylate cyclase activator) or isobutylmethylxanthine (phosphodiesterase inhibitor) increased the rate of spontaneous discharges and decreased the responsiveness to stretch in decapsulated frog muscle spindles (Sokabe et al., 1987). Similar changes in the afferent discharges were observed with application of carbonyl cyanide chlorophenylhydrazone which would increase intraaxonal Ca2+ through uncoupling mitochondria. Application of quercetin (Ca2+ pump inhibitor) resulted in a prolonged increase in the discharge rate, lasting 15–20 s after the release of stretch, These results suggest a certain cascade between cAMP and intracellular Ca2+ in the frog muscle spindle. The present work deals with the effects of modulators of intracellular Ca2+ in order to confirm the contribution of intracellular Ca2+ to the mechano-sensory encoding.
KeywordsMuscle Spindle Spontaneous Discharge External Calcium Afferent Discharge Stretch Response
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