Changes in tonicity of perfusion medium cause prolonged opening of calcium channels of the rat chromaffin cells to evoke explosive secretion of catecholamines.

Secretion of catecholamines (CA) from the isolated rat adrenal gland during and after perfusion with hypertonic Krebs bicarbonate solution was studied. Perfusion with hypertonic solution made by adding either 156 mM sodium chloride, choline chloride, arginine hydrochloride, sodium thiocyanate, or 312 mM sucrose had no effect on the spontaneous secretion of CA. The secretion evoked by splanchnic nerve stimulation (10 Hz for 30 sec) and nicotine (2 micrograms) remained unaffected during perfusion with hypertonic Krebs solution (156 mM excess NaCl). After perfusion of the adrenal gland with hypertonic Krebs solution for 1 hr, if the medium was switched to normal Krebs solution the secretion of CA increased from about 15 to 450 ng; it remained elevated for over 1 hr and eventually returned to the control level after 150 min. Secretion of CA obtained in normal Krebs solution after perfusion of the adrenal gland with hypertonic medium was not affected by atropine plus hexamethonium, splanchnectomy, or tetrodotoxin. After perfusion of the adrenal gland with Ca-free hypertonic (275 mM NaCl) Krebs solution for 1 hr, if the medium was changed to normal Krebs solution the secretion of CA increased from 10 to about 4000 ng in the first 10 min. The enhanced secretion was associated with an almost 70% reduction in the CA content of the adrenal medulla. A massive secretion of CA obtained after switchover from hypertonic to normotonic solution was not associated with an increase in lactate dehydrogenase content in the perfusate, nor was there any reduction in the lactate dehydrogenase content of the adrenal medulla. Immediately after the switchover from hypertonic to normotonic medium, the accumulation of Ca45 increased 6-fold over the control accumulation. The increase in Ca45 accumulation was detected as late as 120 min after the switchover, and was accompanied by an increase in the secretion of CA. The most likely explanation for these findings is that Ca channels, once opened after the switchover, inactivate very slowly and allow Ca ions to flow inside the chromaffin cells. The accumulation of tetraphenylphosphonium (C14-TPP), a marker for the membrane potential, was 86 pg/mg in the adrenal medulla perfused with normal Krebs solution, and decreased to 32 pg/mg in the presence of 55 mM K-Krebs solution. After perfusion with hypertonic medium, the accumulation of C14-TPP increased to about 190 pg/mg. Reperfusion with normal Krebs solution decreased the accumulation to about 105 pg/mg. The increase in C14-TPP accumulation observed in hypertonic medium was prevented by inclusion of 30 mM K in the medium.(ABSTRACT TRUNCATED AT 400 WORDS)