Membrane and secretory properties of renal juxtaglomerular granular cells

The differential role of exogenous and endogenous prostacyclin in the control of renin release from dog renal cortical slices

Using a continuous superfusion system of dog renal cortical slices, we studied the role of prostacyclin in the control of renin release. Superfusate renin activity and prostacyclin as 6-keto-prostaglandin F1alpha, a stable metabolite of prostacyclin, concentrations were measured by radioimmunoassay. Exogenous prostacyclin (0.1, 1, 10 microM) produced a concentration dependent and significant increase in renin release. The calcium ionophore A23187 (10 microM) produced a significant increase in 6-keto-prostaglandin F1alpha release and a significant decrease in renin release. A23187 (10 microM) hardly produced changes of 6-keto-prostaglandin F1alpha release and renin release in the absence of Ca2+. Pretreatment with indomethacin (10 microM) completely abolished the stimulatory effect of A23187 (10 microM) on 6-keto-prostaglandin F1alpha release. On the other hand, the inhibitory effect of A23187 on renin release in the pretreatment with indomethacin was almost equal to that in the "untreatment" with indomethacin. Moreover, we found that there was no association of 6-keto-prostaglandin F1alpha liberation and renin activity. These results indicate that exogenous prostacyclin promotes renin release, and suggest that renin release is not to be modulated by A23187-induced prostacyclin synthesis in dog renal cortical slices.

ATP-dependent mechanism for coordination of intercellular Ca2+ signaling and renin secretion in rat juxtaglomerular cells

A change in intracellular Ca2+ is considered to be the common final signaling pathway through which renin secretion is governed. Therefore, information relating to the generation, control, and processing of Ca2+ signaling in juxtaglomerular cells (JG) will be critical for understanding JG cell behavior. In this study, we investigated the means by which JG cells harmonize their intracellular Ca2+ signals and explored the potential role of these mechanisms in renin secretion. Mechanical stimulation of a single JG cell initiated propagation of an intercellular Ca2+ wave to up to 11.9+/-4.1 surrounding cells, and this was prevented in the presence of the ATP-degrading enzyme, apyrase (1.7+/-0.7 cells), or by desensitization of purinergic receptors via pretreatment of cells with ATP (1.8+/-0.9 cells), thus implicating ATP as a mediator responsible for the propagation of intercellular Ca2+ signaling. Consistent with this, JG cells were demonstrated not to express the gap junction protein connexin43, and neither did they possess functional gap junction communication. Furthermore, massive mechanical stretching of JG cells elicited a 3-fold increase in ATP release. Administration of ATP into isolated perfused rat kidneys induced a rapid, potent, and persistent inhibition of renin secretion, together with a transient elevation of renal vascular resistance. ATP (1 mmol/L) caused up to 79% reduction of the renin secretion activated by lowering the renal perfusion flow (P<0.01). Taken together, our results indicate that under mechanical stimulation, ATP functions as a paracellular mediator to regulate renin secretion, possibly through modulating intra- and intercellular Ca2+ signals.

The role of Ca2+ in the control of renin release from dog renal cortical slices

Using a continuous superfusion system of dog renal cortical slices, we studied the role of Ca(2+) in the intracellular control mechanism for renin release. The calcium ionophore A23187 (10 microM) produced a significant decrease in renin release. This effect was abolished in the absence of extracellular Ca(2+). Moreover, pretreatment with the calmodulin inhibitor W-7 (N-(6-aminohexyl)-5-chloro-1-naphthalenesulfonamide, 20 microM) completely prevented the inhibitory effect of A23187 (10 microM). The beta-adrenoceptor agonist isoproterenol (1, 10 and 100 microM) produced a concentration-dependent increase in renin release. Pretreatment with W-7 (20 microM) potentiated the stimulation of renin release induced by isoproterenol (1 microM). These results suggest that A23187-induced inhibition of renin release is mediated by the activation of calmodulin via an increase in intracellular Ca(2+) and beta-adrenoceptor-stimulated renin release is modulated by intracellular Ca(2+) mobilization.

Role of nitric oxide in the control of renin secretion

Because of the significant constitutive expression of NO synthases in the juxtaglomerular apparatus, nitric oxide (NO) is considered as a likely modulator of renin secretion. In most instances, NO appears as a tonic enhancer of renin secretion, acting via inhibition of cAMP degradation through the action of cGMP. Depending on as yet unknown factors, the stimulatory effect of NO on renin secretion may also switch to an inhibitory one that is compatible with the inhibition of renin secretion by cGMP-dependent protein kinase activity. Whether NO plays a direct regulatory role or a more permissive role in the control of renin secretion remains to be answered.

Role of cGMP-kinase II in the control of renin secretion and renin expression

To investigate the roles of the cGMP-dependent protein kinases (cGKs) in the control of the renin system, we studied the regulation of renin in cGKI- or cGKII-deficient mice in vivo and in vitro. Renal renin mRNA levels both under stimulatory (low-salt diet plus ramipril) and inhibitory (high-salt diet) conditions were not different between wild-type and cGKI-/- mice, but were significantly elevated in cGKII-/- mice under all experimental conditions. In primary cultures of renal juxtaglomerular cells (JG) established from wild-type, cGKI-/-, and cGKII-/- mice, the adenylate cyclase activator forskolin stimulated renin secretion similarly in all genotypes tested. 8-bromo-cGMP attenuated basal and forskolin-stimulated renin secretion in cultures from wild-type and cGKI-/-, but had no effect in cells isolated from cGKII-/- mice. Activation of cGKs by 8-bromo-cGMP decreased renin secretion from the isolated perfused rat kidney, independent of prestimulation by beta-adrenoreceptor activation, macula densa inhibition, reduced perfusion pressure, or by a nominally calcium-free perfusate. Taken together, these findings suggest that activation of cGKII has a general inhibitory effect on renin secretion from renal JG cells.