Renin secretion is regulated by coordinated signaling between the various cells of the juxtaglomerular apparatus. The renin-secreting cells (RSC), which play a major role in the control of blood pressure, are coupled to each other and to endothelial cells by Connexin40 (Cx40)-containing channels. In this study, we show that Cx40 knockout (Cx40-/-) mice, but not their heterozygous littermates, are hypertensive due to the increase in the number of RSC, renin biosynthesis, and plasma renin. Treatment with the angiotensin II receptor AT1 antagonist candesartan or the angiotensin II-converting enzyme inhibitor ramipril reduced the blood pressure of the Cx40-/- mice to the same levels seen in wild-type (WT) mice. The elevated blood pressure of the knockout mice was not affected by clipping one renal artery (2K1C, renin-dependent model of hypertension) or after a high salt diet. Under these conditions, however, Cx40-/- mice showed an altered production and release of renin. The renin mRNA ratio between the clipped and the non-clipped kidney was lower in the knockout than in the WT 2K1C mice. This indicates that the response to a change in blood pressure was altered. The RSC of the Cx40-/- mice did not have a compensatory increase in the levels of either Cx43 or Cx37. Our data show that renin secretion is dependent on Cx40 and suggest the Cx40-/- mice may be a genetic model of renin-dependent hypertension.

Diabetes is associated with significant changes in plasma concentrations of lipoproteins. We tested the hypothesis that lipoproteins modulate the function and survival of insulin-secreting cells. We first detected the presence of several receptors that participate in the binding and processing of plasma lipoproteins and confirmed the internalization of fluorescent low density lipoprotein (LDL) and high density lipoprotein (HDL) particles in insulin-secreting ␤-cells. Purified human very low density lipoprotein (VLDL) and LDL particles reduced insulin mRNA levels and ␤-cell proliferation and induced a dose-dependent increase in the rate of apoptosis. In mice lacking the LDL receptor, islets showed a dramatic decrease in LDL uptake and were partially resistant to apoptosis caused by LDL. VLDLinduced apoptosis of ␤-cells involved caspase-3 cleavage and reduction in the levels of the c-Jun N-terminal kinase-interacting protein-1. In contrast, the proapoptotic signaling of lipoproteins was antagonized by HDL particles or by a small peptide inhibitor of c-Jun N-terminal kinase. The protective effects of HDL were mediated, in part, by inhibition of caspase-3 cleavage and activation of Akt/protein kinase B. In conclusion, human lipoproteins are critical regulators of ␤-cell survival and may therefore contribute to the ␤-cell dysfunction observed during the development of type 2 diabetes.

In adrenal glomerulosa cells, angiotensin II (Ang II) and potassium stimulate aldosterone synthesis through activation of the calcium messenger system. The ratelimiting step in steroidogenesis is the transfer of cholesterol to the inner mitochondrial membrane. This transfer is believed to depend upon the presence of the steroidogenic acute regulatory (StAR) protein. (Ang II) 1 and K ϩ act as regulators of aldosterone synthesis and secretion in adrenal glomerulosa cells. The crucial role of the Ca 2ϩ messenger in the acute regulation of aldosterone production is firmly established (1-5). Indeed, the steroidogenic response of isolated adrenal cells to Ang II and K ϩ is highly dependent upon extracellular Ca 2ϩ concentration (6) and can be blocked by inhibitors of Ca 2ϩ influx across the plasma membrane (4). Moreover, calmodulin antagonists have been shown to inhibit Ang IIstimulated aldosterone production in zona glomerulosa cells (7).Traditionally, aldosterone biosynthesis is functionally divided into three consecutive phases. (i) In the early mitochondrial steps, cholesterol is transported from intracellular lipid droplets into the outer mitochondrial membrane (OM) and then to the inner mitochondrial membrane (IM). The latter step represents the rate-limiting process in all steroidogenic pathways (8) and is followed by the conversion of cholesterol to pregnenolone by the cytochrome P450 scc enzyme. (ii) The intermediate steps take place on the endoplasmic reticulum and involve the conversion of pregnenolone to progesterone by 3␤-hydroxysteroid dehydrogenase isomerase and then to 11-deoxycorticosterone. (iii) The late steroidogenic steps are localized back in the mitochondria and include the formation of corticosterone and its conversion to aldosterone by cytochrome P450 11␤ .The regulation of intramitochondrial cholesterol transfer by cAMP-dependent mechanisms has been extensively studied (9). While the transport of cholesterol from lipid droplets to the outer mitochondrial membrane was found not to be affected by inhibitors of protein synthesis in ACTH-stimulated adrenal

Addition of GnRH to pituitary gonadotrophs preloaded with Quin 2 resulted in a rapid (approximately 8 s) mobilization of an ionomycin-sensitive intracellular Ca2+ pool. A second component of Ca2+ entry via voltage dependent channels contributed about 45% of the peak cytosolic free Ca2+ concentration ([Ca2+]i). Thereafter, influx of Ca2+ via voltage-sensitive and -insensitive channels is responsible for maintenance of elevated [Ca2+]i during the second phase of GnRH action. Addition of inositol 1,4,5-trisphosphate (IP3) to permeabilized pituitary cells resulted in a Ca2+ transient, released from a nonmitochondrial pool, which maintained ambient free Ca2+ concentration around 170 nM in an ATP-dependent mechanism. Successive stimulations of the cells with IP3 produced an attenuated response. Elevation of the gonadotroph [Ca2+]i by ionomycin, to levels equivalent to that induced by GnRH, resulted in LH release amounting to only 45% of the response to the neurohormone. Activation of the voltage-dependent Ca2+ channels by the dihydropyridine Ca2+-agonist [methyl 1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyridine- 5-carboxylate (BAYK8644)] stimulated LH release, 36% of the GnRH (100 nM) response being reached by 10(-8) M of the drug, both [Ca2+]i elevation and GnRH-induced LH release were inhibited similarly (40-50%) by the dihydropyridine Ca2+-antagonist nifedipine. The results indicate that peak [Ca2+]i induced by GnRH in pituitary gonadotrophs is derived mainly from ionomycin-sensitive cellular stores most likely via IP3 formation.(ABSTRACT TRUNCATED AT 250 WORDS)