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B J Whitehouse and D R E Abayasekara


The role played by cyclic AMP (cAMP)-dependent protein kinases (PKAs) in rat adrenal steroidogenesis has been investigated using cAMP analogues which show partial selectivity for the type I and type II PKA isoenzymes. These were aminohexylamino-cAMP (AHA-cAMP; selective for site 1 on type I PKA), N6-benzoyl-cAMP (BZ-cAMP; selective for site 2 on PKA types I and II) and 8-thiomethyl-cAMP (TM-cAMP; selective for site 1 on type II PKA). Positive cooperativity exists between the two nucleotide-binding sites, thus the presence of type I PKA was inferred when synergistic increases in corticosteroid production were obtained with AHA-cAMP plus BZ-cAMP and that of type II PKA when synergistic increases were obtained with TM-cAMP plus BZ-cAMP.

The effects of AHA-cAMP, TM-cAMP and BZ-cAMP (10–100 μmol/l) on aldosterone production by glomerulosa cell preparations and corticosterone production by fasciculata/reticularis cell preparations were compared. Dose-related stimulation of steroid production was obtained with each cAMP analogue in both types of cell preparation. Experiments were performed using the cAMP analogues in combination at doses which gave minimal stimulation individually. Cells were incubated with AHA-cAMP (66 and 100 μmol/l) or TM-cAMP (15, 30 and 45 μmol/l) in the presence and absence of 15μmol BZ-cAMP/l. Synergistic responses were obtained with both analogue pairs in both cell types. The synergism ratio in fasciculata/reticularis cell preparations for the type I PKA selective pair of analogues (100 μmol AHA-cAMP/l plus 15μmol BZ-cAMP/l) was significantly higher (P<0·01) than that for the type II selective pair (45μmol TM-cAMP/l plus 15μmol BZ-cAMP/l; 7·9±1·2 (mean±s.e.m.) and 2·6±0·3 respectively). In zona glomerulosa preparations the ratio was higher (P<0·05) for the type II selective pair (1·6±0·1 for AHA-cAMP plus BZ-cAMP and 2·8±0·4 for TM-cAMP plus BZ-cAMP).

The effects of 100μmol AHA-cAMP/l and 45μmol TM-cAMP/l on the response to ACTH (1 pmol/l–10 nmol/l) were examined. Synergistic responses were obtained in fasciculata/reticularis cells with both analogues in combination with low concentrations of ACTH (10 and 100 pmol/l). In zona glomerulosa cells only the addition of TM-cAMP (45 μmol/l) in combination with 10 pmol ACTH/1 gave rise to synergistic increases in aldosterone production, which suggests that there may be some compartmentalization of the cAMP-dependent pathway in these cells.

The results indicate that both isoenzymes of PKA are present in rat adrenocortical cells and can play a part in the control of steroidogenesis. Type I PKA activity appears dominant in the control of zona fasciculata/reticularis cell function whereas modulation of type II PKA activity plays a more significant role in the responses of zona glomerulosa cells.

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G. P. Vinson, S. M. Laird, B. J. Whitehouse, and J. P. Hinson


Recent data have implicated the phosphatidylinositol/calcium second-messenger system in the control of aldosterone secretion by the adrenal zona glomerulosa. However, in the rat adrenal there are few reports of a direct effect of protein kinase C activation on steroid secretion, while the effects of calcium mobilization may be variable. Since the rat adrenal zona glomerulosa is sensitive to the mode of tissue preparation, these mechanisms were reinvestigated in intact (non-dispersed) capsular tissue and collagenase-dispersed zona glomerulosa cells.

Steroidogenesis in the intact zona glomerulosa was markedly affected by agonists of the calcium messenger system. Most notably, aldosterone and 18-hydroxycorticosterone (18-OH-B) secretion were stimulated by A23187 (100 nmol to 10 μmol/l) and BAY K 8644 (500 nmol/l). Phorbol 12-myristate 13-acetate (TPA; 1 pmol to 1 μmol/l) stimulated aldosterone secretion at all doses and caused a dose-dependent increase in 18-OH-B and 18-hydroxydeoxycorticosterone (18-OH-DOC) secretion. Corticosterone secretion was slightly increased in the presence of A23187 but not by TPA or BAY K 8644. Production of 18-OH-DOC was unaffected by A23187 and BAY K 8644. The calcium channel antagonist verapamil (10 μmol/l) inhibited ACTH-stimulated aldosterone secretion by the intact zona glomerulosa but had no effect on corticosterone secretion.

Verapamil (10 μmol/l) also inhibited the increase in aldosterone secretion by collagenase-dispersed zona glomerulosa cells stimulated by ACTH (100 fmol to 100 nmol/l), angiotensin II (100 pmol to 10 nmol/l) and potassium (5·9 and 8·4 mmol/l); stimulated corticosterone secretion was unaffected. Aldosterone secretion by dispersed zona glomerulosa cells was unaffected by the calcium ionophore A23187 (10 nmol to 100 μmol/l) or by TPA (1 nmol to 10 μmol/l). Corticosterone secretion was also unaffected by A23187 over the same dose range, but was increased slightly by high doses of TPA (10 and 1 μmol/l), while the calcium channel agonist BAY K 8644 had no effect on either steroid.

The results show that calcium mobilization and protein kinase C activation can stimulate steroid secretion in the rat adrenal zona glomerulosa and that large and reproducible effects are seen when intact tissue is used. In general only 18-OH-B and aldosterone secretion were affected; effects on corticosterone and 18-OH-DOC were much less marked. Together with the effects of calcium blockers, the data strongly support the view that protein kinase C activation and calcium mobilization are primarily involved in the control of the 'late pathway' of aldosterone biosynthesis.