Exocytotic Secretion of Catecholamines From the Cat Adrenal Medulla by Sodium Deprivation: Involvement of Calcium Influx Mechanism

Abstract

Cat adrenal glands were perfused with Ca‐deficient medium and secretion of catecholamines (CA) was induced by perfusion with Na‐free medium in which NaCl was replaced by an osmotically equivalent amount of sucrose. Release of CA and dopamine‐β‐hydroxylase (DBH), but not that of phenylethanolamine‐N‐methyltransferase, was concomitantly found in the effluents when the adrenals were stimulated, indicating that secretion was due to exocytosis. Secretion of CA induced by Na‐free (sucrose) medium was dependent on the concentration of Ca and was saturated at 0.5 mm of Ca. Sr or Ba substituted for Ca in maintaining secretion by Na‐free (sucrose) medium. The addition of Na, Li or alkali metal ions to Na‐free (sucrose) medium containing Ca reduced the response to a variable extent but this inhibition was reversed by raising the concentration of Ca in the Na‐free medium. All of the Na substitutes used induced secretion only when this medium contained Ca. However, different Na substitutes released different amounts of CA; sucrose was most effective, K, Tris and choline were moderately and Li least effective. Secretion of CA by Na‐free (sucrose) medium was strongly inhibited by D‐600, tetracaine or divalent cations such as Co, Ni, Zn and Mg. The inhibition by Co was partially reversed by raising the concentration of Ca in the Na‐free medium. Secretion of CA from bovine isolated chromaffin cells was induced by Na‐deficient (sucrose) medium and was dependent on the concentrations of ionized Ca involved. All the Na substitutes tested increased secretion of CA and 45Ca uptake, in a parallel fashion. A correlation between secretion and 45Ca uptake was found under various experimental manipulations which reduced secretion of CA. These results demonstrated that unlike the perfused bovine adrenals, the Ca influx mechanism is essential for secretion by Na deprivation in the perfused cat adrenals as it is in bovine isolated chromaffin cells. It is suggested that Na deprivation increases Ca entry through the Ca channels by eliminating the competition between Na and Ca, and possibly by activating Ca influx linked with Na efflux.

A small increment of external K concentration accelerates adrenomedullary catecholamine secretion induced by ouabain and metabolic inhibitors.

“…The rate of 45Ca efflux from the medullary slices (RINK, 1977) and the perfused cortex-free medulla (AGUIRRE et a!., 1977) was largely dependent on external Na. Conversely, all the procedures leading to a rise in the concentration of internal Na was found to increase Ca influx in exchange for internal Na (that is, Na-dependent Ca influx) thereby increasing catecholamine (CA) secretion (ESQUERRO et al, 1980;GARCIA et al, 1980GARCIA et al, , 1981bNISHIMURA et al, 1981;SORIMACHI et al, 1981;SORIMACHI and YAMAGAMI, 1983). Recently, GARCIA et al (1981a) reported that ouabain treatment of the adrenal gland enabled a small increment of external K to increase markedly the rate of CA secretion.…”

Catecholamine release evoked by lithium from the perfused adrenal gland of the cat

“…This Na-Ca exchange system might be involved in the control of the intracellular concentration of ionized Ca levels and, therefore, in the modulation of catecholamine release by chromaffin cells. Procedures leading to an increase in the ratio [Na]i/[Na]l such as Na deprivation or ouabain treatment, will activate this system favouring the entry of Ca into the cell, in exchange for internal Na Nishimura et al, 1981;. Then the elevated intracellular [Ca] will result in a parallel increase of catecholamine output by the gland.…”

Inhibition of Na pumps enhances Ca-dependent release of vasopressin from the isolated neurohypophysis of the rat.

“…The response was reversibly diminished 10 min after exposure to a medium containing 5 mM K. These results are essentially similar to those obtained in the chromaffin cells, where all the procedures leading to an expected rise in internal Na increased catecholamine secretion in response to Ca reintroduction (ESQUERRO et al, 1980;GARCIA et al, 1981;SORIMACHI et al, 1981). We have therefore tested the effects of D 600 (methoxyverapamil) and Co ions on the responses to Ca after ouabain treatment, since catecholamine secretion in response to Ca reintroduction with or without simultaneous removal of external Na was sensitive to these Ca channel blockers in the chromaffin cells (AGUIRRE et al, 1979;NISHIMURA et al, 1981;Fig. 2.…”