EVIDENCE FOR THE INVOLVEMENT OF CHROMAFFIN CELLS IN THE STIMULATORY EFFECT OF TACHYKININS ON CORTICOSTEROID SECRETION BY THE FROG ADRENAL-GLAND

The adrenal gland of the frog is innervated by a network of fibers con taining two tachykinins (ranakinin and [Leu(3),Ile(7)]neurokinin A), w hich both stimulate corticosteroid secretion from frog adrenal tissue. The aim of the present study was to determine the mode of action of t achykinins on the frog adrenal gland. Double immunolabeling of tissue sections with a monoclonal antibody to tyrosine hydroxylase and an ant iserum to substance P showed that tachykinin-containing fibers are pre ferentially apposed onto chromaffin cells. Immunocytochemical labeling at the electron microscope level revealed that tachykinin-immunoreact ive fibers establish close contacts only with adrenochromaffin cells. Ranakinin stimulated corticosterone and aldosterone secretion from per ifused adrenal slices, but had no stimulative effect on dispersed adre nal cells. Cytoautoradiographic labeling of frog adrenal cells in prim ary culture with [H-3]substance P revealed the existence of specific b inding sites located exclusively on chromaffin cells. Microfluorimetri c measurement of cytosolic calcium concentrations ([Ca2+](i)) in cultu red adrenal cells showed that ranakinin induced a dose-dependent incre ase in [Ca2+](i) in chromaffin cells (ED(50) = 2 x 10(-7) M). In contr ast, ranakinin did not affect [Ca2+](i) in adrenocortical cells. The p resent results indicate that in the frog adrenal gland, tachykinin-con taining fibers make preferential contacts with chromaffin cells, and t achykinins directly activate chromaffin cells. These data suggest that the stimulative effect of tachykinins on corticosteroid secretion is mediated via presynaptic activation of adrenochromaffin cells.