Operation of internal Na-dependent Ca influx mechanism associated with catecholamine secretion in the adrenal chromaffin cells.

Abstract

The cat adrenal gland was retrogradely perfused with a medium lacking divalent cation, and the secretion of catecholamines was induced by Ca reintroduction with or without simultaneous removal of external Na. These responses were markedly reduced after prior exposure to a medium containing Mg (1 mM) or lacking Na. The inhibition caused by Mg was reversed by ouabain treatment. Reversing the concentration gradient for Na alone by replacing external Na with Tris, choline, or Li did not stimulate catecholamine secretion when Ca2+ (0.1 mM) was present in the external medium throughout the experiment. However, the treatment with ouabain reversed this inhibitory effect of Ca so that Na removal did induce marked secretion. Essentially similar results concerning secretion were obtained in the cultured bovine chromaffin cells. There was a reasonable correlation between secretion and 45Ca uptake in these cells under various experimental manipulations, and alterations of these parameters were well correlated with the level of internal Na. Furthermore, the rate constant of 22Na efflux was found to increase when Ca was reintroduced with the simultaneous removal of Na during exposure to a medium containing ouabain. These results are consistent with the view that the mechanism of internal Na-dependent Ca influx is activated to induce catecholamine secretion whenever internal Na is raised above a critical level. On the other hand, there were significant increases in the catecholamine secretion and 45Ca uptake which were induced by substitution of NaCI with sucrose, even when the operation of the internal Nadependent Ca influx mechanism was markedly restricted by various experimental manipulations. This suggests that other secretory mechanisms are involved under these conditions. This would account for the largest secretory effect of sucrose substitution under the condition in which internal Na-dependent Ca influx is activated.

Tumor promoters potentiate the adrenomedullary secretion induced by acetylcholine and excess K possibly by stimulating phorbol ester receptors.

“…On the other hand, the responses to histamine (0.5 mM) or Ca (0.5 mM) reintroduction during exposure to a medium lacking divalent cation (DOUGLAS and RUBIN, 1963;SORIMACHI and NISHIMURA, 1984) were little affected by TPA (55±11 % in case of histamine, N=3, and 52±17% of the preceding controls in case of Ca reintroduction, N= 3). These results raise the possibility that TPA somehow activates voltage-dependent Ca channels as does dihydropyridine BAY-K-8644 (GARCIA et al, 1984).…”

Potentiation by Bay-K-8644 of the adrenal catecholamine secretory response to Ca re-introduction and ouabain: Possible activation of Ca influx linked with Na efflux.

“…Previous results have indicated that the inhibition of Na pump overcame the inhibitory effect of Mg treatment, so that Ca re-introduction caused substantial secretion. This restored response was believed to be due to activation of Ca influx mechanism linked with Na efflux because of the increase in the concentration of internal Na (ESQUERRO et al, 1980;GARCIA et a!.,1981;SORIMACHI and YAMAGAMI, 1983;SORIMACHI and NISHIMURA, 1984). To see whether or not Bay-K-8644 potentiates the secretion due to this Ca influx mechanism, we investigated the effect of this agent on the response to Ca re-introduction under conditions in which Na pumping activity was inhibited by K deprivation, ouabain, or a metabolic inhibitor (KCN).…”

“…These results led to the proposal that secretion was induced by Ca influx linked with Na efflux (ESQUERRO et al, 1980;GARCIA et al, 1980GARCIA et al, , 1981SORIMACHI et al, 1981;SORIMACHI and NISHIMURA, 1984). On the other hand, this response is abolished by organic (methoxy-verapamil and nifedipine;AGUIRRE et a!., 1979;NISHIMURA et al, 1981;MONTIEL et al, 1984;SORIMACHI and NISHIMURA, 1984) and inorganic blockers (Co, Mn, and Zn;NISHIMURA et al, 1981) of the voltage-dependent Ca channel, and thus led to the suggestion that the Ca entry pathway linked with Na efflux has pharmacological characteristics quite similar to those of the voltage-dependent Ca channel (AGUIRRE et al, 1979;SORIMACHI and NISHIMURA, 1984). However, it has recently been found that both responses to Ca re-introduction and to ouabain in the presence of Ca and Mg were greatly raise the fundamental question as to whether or not Ca entry linked with Na efflux actually occurs through the pathway distinct from the voltage-dependent Ca channel.…”

“…The inhibitory effect of Mg treatment on secretion was overcome by inhibiting the Na pump with ouabain, K deprivation, or metabolic inhibitors, and the restored responses under these conditions were dependent on the concentration of external Na (ESQUERRO et al, 1980;GARCIA et al, 1981;SORIMACHI et al, 1981;SORIMACHI and YAMAGAMI, 1983;SORIMACHI and NISHIMURA, 1984). These results led to the proposal that secretion was induced by Ca influx linked with Na efflux (ESQUERRO et al, 1980;GARCIA et al, 1980GARCIA et al, , 1981SORIMACHI et al, 1981;SORIMACHI and NISHIMURA, 1984).…”

“…The inhibitory effect of Mg treatment on secretion was overcome by inhibiting the Na pump with ouabain, K deprivation, or metabolic inhibitors, and the restored responses under these conditions were dependent on the concentration of external Na (ESQUERRO et al, 1980;GARCIA et al, 1981;SORIMACHI et al, 1981;SORIMACHI and YAMAGAMI, 1983;SORIMACHI and NISHIMURA, 1984). These results led to the proposal that secretion was induced by Ca influx linked with Na efflux (ESQUERRO et al, 1980;GARCIA et al, 1980GARCIA et al, , 1981SORIMACHI et al, 1981;SORIMACHI and NISHIMURA, 1984). On the other hand, this response is abolished by organic (methoxy-verapamil and nifedipine;AGUIRRE et a!., 1979;NISHIMURA et al, 1981;MONTIEL et al, 1984;SORIMACHI and NISHIMURA, 1984) and inorganic blockers (Co, Mn, and Zn;NISHIMURA et al, 1981) of the voltage-dependent Ca channel, and thus led to the suggestion that the Ca entry pathway linked with Na efflux has pharmacological characteristics quite similar to those of the voltage-dependent Ca channel (AGUIRRE et al, 1979;SORIMACHI and NISHIMURA, 1984).…”