Phorbol esters, vasopressin and angiotensin II block alpha 1-adrenergic action in rat hepatocytes. Possible role of protein kinase C

Mechanical load opposes angiotensin-mediated decrease in vascular alpha 1-adrenoceptors

alpha 1-Adrenergic receptor contraction of vascular smooth muscle is augmented by increases in angiotensin II and also in several forms of hypertension. Whether angiotensin directly modulates alpha 1-adrenoceptor subtype expression to contribute to this effect is unknown. In a previous study, we demonstrated that increased mechanical load (pressure) per se does not alter expression of alpha 1B- and alpha 1D-adrenoceptors in rat aortic smooth muscle in cell culture, in vitro or in vivo. However, findings in aortic coarctation hypertension suggested that a humoral factor, possibly angiotensin, selectively reduces alpha 1B-adrenoceptors and that increased mechanical load opposes this decrease. The present study examined this hypothesis by determining the effect of angiotensin alone and in the presence of mechanical loading on the expression of alpha 1D- and alpha 1B-adrenergic receptor mRNAs and alpha 1-receptor density in cultured aortic smooth muscle cells. alpha 1D mRNA content, per smooth muscle cell, concentration-dependently decreased after 3 hours of exposure to 0.3 nmol/L to 1 mumol/L angiotensin but by 24 hours had returned to control levels. In contrast, alpha 1B mRNA concentration-dependently declined at a later time (24 hours) and remained decreased at 48 hours to 27 +/- 6% of control with 1 mumol/L angiotensin. Angiotensin also decreased alpha 1-adrenoceptor density in a dose-dependent manner. Angiotensin had no effect on cell number in these confluent, quiescent cells but did increase cell protein and total RNA. This cellular hypertrophy and the decreases in alpha 1-adrenoceptor mRNAs were blocked by the angiotensin type 1 receptor antagonist losartan. Cyclic mechanical loading of smooth muscle cells opposed the angiotensin-mediated hypertrophy and decrease in alpha 1B mRNA expression and alpha 1-adrenergic receptor density. These data suggest that angiotensin and intravascular pressure interact to affect cell growth and expression of alpha 1B-adrenergic receptors by vascular smooth muscle.

Metabolic regulation by alpha 1- and alpha 2-adrenoceptors

The role of alpha-adrenoceptors in the mediation of autonomic function, particularly in the control of the cardiovascular system, is widely known. However, alpha-adrenoceptors are also important in the regulation of a variety of metabolic processes that occur in the body either through direct action or by stimulation of the release of other mediators that control metabolic function. Thus, alpha 2-adrenoceptor activation by circulating or neuronally released catecholamines inhibits the release of insulin from pancreatic islet beta-cells and, by inhibiting this response, alpha 2-adrenoceptor antagonists have been shown to have an antihyperglycemic effect. The alpha-adrenoceptor-mediated regulation of the release of pituitary hormones is indirect, with alpha-adrenoceptors being located on peptidergic neurons in the hypothalamus that secrete releasing hormones into the hypophysial portal system to regulate the secretion of hormones from the anterior pituitary gland. Thus, the increase in cortisol secretion from the adrenal glands following a meal is produced, at least in part, by an alpha 1-adrenoceptor-mediated increase in vasopressin and CRF-41 secretion from neurons on the hypothalamus that stimulate the release of adrenocorticotrophic hormone secretion from the pituitary gland, which subsequently stimulates the synthesis and release of cortisol from the adrenal medulla. In addition to metabolic regulation by alpha 1- and alpha 2-adrenoceptors within the endocrine system, alpha-adrenoceptors are also a component of the system that regulates certain aspects of metabolism within autonomic effector cells, such as the control of smooth muscle cell division and growth during periods of continued alpha-adrenoceptor activation as a result of activation of second messenger systems.

Phorbol esters and calcium-mobilizing hormones increase membrane-associated protein kinase C activity in rat hepatocytes

Vasopressin, angiotensin II, epinephrine (alpha 1-adrenergic action) and phorbol 12-myristate 13-acetate (PMA) induce increases in membrane-associated protein kinase C activity concomitant with decreases in the cytosolic activity. The data indicate that the calcium-mobilizing hormones and the active phorbol ester induce translocation from the cytosol to the plasma membrane of this protein kinase. The protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine, blocked the translocation to the membrane of this protein kinase induced by PMA and vasopressin.

Action of phenylephrine on protein synthesis in liver cells

The alpha-adrenergic agonist phenylephrine was found to inhibit protein labelling from [3H]valine in isolated liver cells. This effect is only observable under conditions of partial Ca2+ depletion and in cells displaying maximal rates of protein labelling, i.e. cells isolated from fed animals or from starved animals when incubated in the presence of alanine. The ability of phenylephrine to inhibit protein labelling at near-saturating concentrations of the amino acid precursor indicates that this alpha-agonist actually decreases the rate of protein synthesis. The possibility that phenylephrine acts by making cellular Ca2+ availability further limiting can be ruled out, since alanine stimulates protein labelling under conditions of severe Ca2+ depletion obtained by pretreatment of the cells with EGTA. The following observations indicate that the phenylephrine action may be mediated by an increase in cellular cyclic AMP content: (1) a close relationship was found between the abilities of phenylephrine to inhibit protein labelling and to increase cyclic AMP content; (2) cyclic AMP mimics the phenylephrine action only in cells partially depleted of Ca2+; (3) the alpha 1-antagonist prazosin, which inhibited the phenylephrine-mediated increase in cyclic AMP, also abolished the effect on protein synthesis.

Inhibitors of diacylglycerol lipase and diacylglycerol kinase inhibit carbamylcholine-stimulated responses in guinea pig pancreatic minilobules

We earlier showed that the diacylglycerol (DG) lipase inhibitor, RHC 80267, increased the steady-state level of DG and inhibited the release of arachidonic acid (AA) in carbamylcholine (CCh)-stimulated pancreatic minilobules (J. F. Dixon and L. E. Hokin, (1984) J. Biol. Chem. 259, 14418-14425). There was no effect on phospholipid metabolism. We have now investigated the effect of RHC 80267 on CCh-stimulated formation of inositol monophosphate formation, cGMP formation, and amylase release. CCh (10 microM) increased cGMP formation by approximately 20-fold, and this response was inhibited 55-75% by RHC 80267 (75-100 microM). RHC 80267 had no effect on either nitroprusside- or calcium ionophore-stimulated cGMP formation, arguing against a direct inhibition of guanylate cyclase by RHC 80267. Arachidonic acid, the release of which is inhibited by RHC 80267, neither stimulated cGMP formation nor reversed the effect of RHC 80267 on CCh-stimulated cGMP formation. This suggests, but does not prove, that the rise in cGMP in response to CCh is not due to an increase in AA as has been suggested. Both phorbol myristate acetate (25 nM) and the DG kinase inhibitor R 59022 (10 microM) inhibited CCh-stimulated cGMP formation by 40%. RHC 80267 also inhibited CCh-stimulated inositol phosphate accumulation and amylase release by 60 and 40%, respectively. The data suggest that the inhibition of CCh-stimulated cGMP formation and other muscarinic responses by RHC 80267 is probably the result of feedback inhibition of the cholinergic receptor via activation of protein kinase C by the elevated DG.

Inhibitors of protein kinase C block the alpha 1-adrenergic refractoriness induced by phorbol 12-myristate 13-acetate, vasopressin and angiotensin II

Vasopressin and angiotensin II inhibited in a dose-dependent fashion the stimulation of ureagenesis induced by alpha 1-adrenergic activation in hepatocytes incubated in medium without calcium and containing 25 microM EGTA. Vasopressin was more potent than angiotensin II. The effect of different inhibitors of protein kinase C on the alpha 1-adrenergic blockade induced by the vasopressor peptides was tested. It was observed that N-(6-aminohexyl)-5-chloro-1-naphthalene sulfonamide (W-7), 4-aminoethyl-1-[2,3-bis(n-decloxyl)-n-propyl]-4-phenylpiperadin e dihydrochloride (CP-46,665-1); 8-(N,N-diethylamino)octyl-3,4, 5-trimethoxybenzoate (TMB-8), polymyxin B and 1-(5-isoquinolynsulfonyl)-2-methylpiperazine (H-7) block this effect of the vasopressor peptides in a dose-dependent fashion. The active phorbol ester, phorbol 12-myristate 13-acetate (PMA), also inhibited the alpha 1-adrenergic stimulation of ureagenesis in these cells. The inhibitors of protein kinase also blocked the effect of phorbol esters but a preincubation with the inhibitors before the addition of PMA was required. alpha 1-Adrenergic activation of phosphatidylinositol labeling was also abolished by PMA; the inhibitors of protein kinase partially blocked this effect of PMA. In summary, our data indicate that inhibitors of protein kinase C can block the alpha 1-adrenergic refractoriness induced by active phorbol esters, vasopressin and angiotensin II. The data are consistent with an important role of protein kinase C in modulating the alpha 1-adrenergic responsiveness of hepatocytes.

Homologous and heterologous desensitization of one of the pathways of the alpha 1-adrenergic action. Effects of epinephrine, vasopressin, angiotensin II and phorbol 12-myristate 13-acetate

Activation of protein kinase C blocks the alpha 1-adrenergic action in hepatocytes. Preincubation of hepatocytes (in buffer with or without calcium) with vasopressin, angiotensin II, phorbol myristate acetate (PMA) or epinephrine + propranolol markedly diminished the alpha 1-adrenergic responsiveness of the cells (stimulation of ureagenesis) assayed in buffer without calcium. On the contrary, when the alpha 1-adrenergic responsiveness was assayed in buffer containing calcium no effect of the preincubation with vasopressin, angiotensin II or PMA was observed. Preincubation with epinephrine diminished the alpha 1-adrenergic responsiveness of the cells. In hepatocytes from hypothyroid rats the preincubation with the activators of protein kinase C (vasopressin, angiotensin II, phorbol 12-myristate 13-acetate and epinephrine) reduced markedly the alpha 1-adrenergic responsiveness of the cells, whereas in identical experiments using cells from adrenalectomized rats only the preincubation with epinephrine diminished the responsiveness. It is concluded that activation of protein kinase C induces desensitization of the alpha 1-adrenergic action in hepatocytes and that the calcium-independent pathway of the alpha 1-adrenergic action (predominant in cells from hypothyroid animals) resensitizes more slowly than the calcium-dependent pathway (predominant in cells from adrenalectomized rats). Epinephrine in addition to inducing this type of desensitization (through protein kinase C) leads to a further refractoriness of the cells towards alpha 1-adrenergic agonists.