It has recently been shown that two novel tachykinins, ranakinin and [Leu3, Ile7]neurokinin A, are present in fibers innervating the frog adrenal gland, and it has been demonstrated that tachykinins stimulate corticosteroid secretion in vitro through activation of chromaffin cells. The purpose of the present study was to investigate the effect of ranakinin on cytosolic free calcium concentrations ([Ca2+]i) and to determine the source of calcium involved. Cultured adrenal cells were loaded with the fluorescent calcium indicator indo-1, and changes in [Ca2+]i were studied using dual emission wavelength microfluorimetry. Administration of a brief pulse of ranakinin (1 μM; 1 sec) in the vicinity of chromaffin cells caused an immediate and transient increase in [Ca2+]i. Repeated pulses of ranakinin resulted in a gradual decline in the [Ca2+]i response, suggesting the occurrence of a desensitization phenomenon. Preincubation of the cells with the calcium channel blockers nifedipine (10 μM) and omega-conotoxin (1 microM) did not alter the response of chromaffin cells to ranakinin. Chelation of extracellular calcium by EGTA (10 mM) caused a marked decrease in the basal [Ca2+]i, but did not suppress the ranakinin-induced [Ca2+]i increase. Conversely, incubation of the cells with thapsigargin (10 microM), an inhibitor of calcium adenosine triphosphatase activity, abolished the stimulatory effect of ranakinin, indicating that the increase in [Ca2+]i can be ascribed to mobilization of calcium from intracellular stores. Preincubation of adrenal cells with the phospholipase C antagonist U-73122 (1 microM; 18 min) or with pertussis toxin (10 μM; 18 h) totally blocked the ranakinin-induced [Ca2+]i rise. Taken together, these data indicate that in frog adrenochromaffin cells, ranakinin causes mobilization of calcium from intracellular stores. The effect of ranakinin is mediated through activation of a phospholipase C via a pertussis toxin-sensitive G protein.
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