We compared the action of K+ on aldosterone secretion from isolated bovine adrenal glomerulosa cells with that of ionophore A23187. Addition of either 50 nM-A23187 or 8 mM-K+ to perifused cells induces a similar initial aldosterone-secretory responses, and a similar sustained increases in Ca2+ entry. However, K+-induced secretion is more sustained than is A23187-induced secretion, even though each agonist appears to act by increasing Ca2+ entry into the cells. When [3H]inositol-labelled cells are stimulated by 8 mM-K+, a small decrease in phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P2] is observed. This decrease is not accompanied by an increase in inositol trisphosphate (InsP3) concentration. Also, if [3H]arachidonic acid-labelled cells are exposed to 8 mM-K+, there is no increase in [3H]diacylglycerol production. When [3H]inositol-labelled cells are stimulated by 50 nM-A23187, a small decrease in PtdIns(4,5)P2 is observed. This decrease is not accompanied by an increase in InsP3. The cyclic AMP content of K+-treated cells was approximately twice that in A23187-treated cells. If cells are perifused simultaneously with 50 nM-forskolin and 50 nM-A23187, the initial aldosterone-secretory response is similar to that induced by A23187 alone, and the response is sustained rather than transient, and is similar to that seen during perifusion of cells with 8 mM-K+. This dose of forskolin (50 nM) causes an elevation of cyclic AMP concentration in A23187-treated cells, to a value similar to that in K+-treated cells. These results indicate that, in K+-treated cells, a rise in cyclic AMP content serves as a positive sensitivity modulator of the Ca2+ message, and plays a key role in mediating the sustained aldosterone-secretory response.

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