Influences of Extracellular Calcium and Potassium Concentrations on Adrenaline Release and Membrane Potential in the Perfused Adrenal Medulla of the Rat

Abstract

Prolonged perfusion of isolated rat adrenal glands with high K produced a transient increase in adrenaline [epinephrine] release that reached a maximum within 1 min and then declined with a half-time of about 5 min. The higher the [K]o used, the larger was the release of adrenaline elicited. There was a linear relation between the mean rate of adrenaline release in the initial 5 min of continuous stimulation with excess K and logarithmic increase in the [K]o. The higher the [Ca]o used, the steeper the relation obtained: the slope determined in the presence of 5mM-Ca was nearly twice as much as that obtained in the presence of 0.5 mM-Ca. The release of adrenaline in response to excess K was virtually abolished by the omission of [Ca]o. The chromaffin cells were depolarized in proportion to logarithmic increase in the [K]o. Linear relations were thus obtained between the mean rates of adrenaline release and the membrane potential of chromaffin cells in the presence of different [Ca]o. Application of high K revived secretory response of a gland after prolonged exposure to acetylcholine in the presence of tetrodotoxin, when adrenaline release had declined below a half of the maximum response. Application of acetylcholine also revived secretory response after prolonged exposure to high K. The mechanism of Ca influx activated by high K may differ from that activated by acetylcholine.