The effects of various concentrations of extracellular K
+
(3.6 - 13 mM) on the steroid (corticosterone and aldosterone) and cyclic AMP outputs of capsular cells (95% zona glomerulosa) of the rat adrenal cortex were studied at different concentrations of extracellular Ca
2+
. Small amounts of EGTA (50 μM) were added to reduce the free Ca
2+
concentrations effectively to zero at the lowest possible total Ca
2+
concentration. At a total extracellular concentration of 2.5 mM Ca
2+
, in 27 experiments the mean values of the steroid and cAMP outputs showed a maximum at 8.4 mM K
+
. The increase in steroid and cAMP outputs at 5.9, 8.4 and 13 mM K
+
compared with that at 3.6 mM were highly significant (
p
< 0.01). The overall correlation of either corticosterone or aldosterone with cAMP outputs was also highly significant and was even better from 3.6 to 8.4 mM K
+
. Lowering the effective free concentration of Ca
2+
to zero decreased the steroid and cAMP outputs significantly at all K
+
concentrations, and no output was then significantly higher than at 3.6 mM. With the pooled data on outputs at all total Ca
2+
(2.5, 0.5, 0.25, 0.10, 0.05 and 0.0 mM) and K
+
(3.6, 5.9, 8.4 and 13 mM) concentrations, the correlation of either steroid with cAMP outputs was highly significant (but again optimally from 3.6 to 8.4 mM K
+
). Nifedipine (10
-6
to 10
-4
M) was added to the incubations with the aim of specifically inhibiting Ca
2+
influx at total extracellular Ca
2+
concentrations of 2.5, 1.25 and 0.25 mM and with the usual K
+
concentrations. The cAMP outputs were reduced at all K
+
concentrations above 3.6 mM K
+
. The effect was highly significant at 10
-4
M nifedipine and a total Ca
2+
of 1.25 mM, which with the incubation conditions used, corresponds to the free Ca
2+
concentrations
in vivo
. These results indicate that cAMP plays a significant role in the stimulation of steroid output by K
+
particularly between 3.6 and 8.4 mM K
+
. In this range of K
+
concentrations the stimulation of cAMP seems to be controlled by increases in Ca
2+
influx. The correlation of steroid and cAMP output at the higher K
+
concentrations (between 8.4 and 13 mM K) and at the various total Ca
2+
concentrations is less significant. Also, with all concentrations of added nifedipine there is an ‘anomalous’ increase in steroid output at 13 mM K
+
and at total Ca
2+
concentrations of 2.5 and 1.25 mM. However, at the same K
+
concentrations and at 0.25 mM Ca
2+
, nifedipine decreases steroid outputs. Our previous data, obtained after addition of maximally effective amounts of cAMP, indicated that there were also non-cAMP mechanisms involved in the stimulation of steroidogenesis by K
+
in z. g. cells. The present data confirm this conclusion, particularly at K
+
concentrations above 8.4 mM. They also indicate that at these higher K
+
concentrations, by non-cAMP mechanisms increasing intracellular Ca
2+
concentrations probably inhibit steroidogenesis. We conclude, however, that in the physiological range of K
+
concentrations, the role of cAMP in zona glomerulosa cells is at least comparable in importance to that of non-cAMP mechanisms.
Guinea pig adrenocortical cells: in vitro characterization of separated zonal cell types.