Angiotensin II type 1 receptor activation modulates L- and T-type calcium channel activity through distinct mechanisms in bovine adrenal glomerulosa cells

T-Type Calcium Channel: A Privileged Gate for Calcium Entry and Control of Adrenal Steroidogenesis

Intracellular calcium plays a crucial role in modulating a variety of functions such as muscle contraction, hormone secretion, gene expression, or cell growth. Calcium signaling has been however shown to be more complex than initially thought. Indeed, it is confined within cell microdomains, and different calcium channels are associated with different functions, as shown by various channelopathies. Sporadic mutations on voltage-operated L-type calcium channels in adrenal glomerulosa cells have been shown recently to be the second most prevalent genetic abnormalities present in human aldosterone-producing adenoma. The observed modification of the threshold of activation of the mutated channels not only provides an explanation for this gain of function but also reminds us on the importance of maintaining adequate electrophysiological characteristics to make channels able to exert specific cellular functions. Indeed, the contribution to steroid production of the various calcium channels expressed in adrenocortical cells is not equal, and the reason has been investigated for a long time. Given the very negative resting potential of these cells, and the small membrane depolarization induced by their physiological agonists, low threshold T-type calcium channels are particularly well suited for responding under these conditions and conveying calcium into the cell, at the right place for controlling steroidogenesis. In contrast, high threshold L-type channels are normally activated by much stronger cell depolarizations. The fact that dihydropyridine calcium antagonists, specific for L-type channels, are poorly efficient for reducing aldosterone secretion either in vivo or in vitro, strongly supports the view that these two types of channels differently affect steroid biosynthesis. Whether a similar analysis is transposable to fasciculata cells and cortisol secretion is one of the questions addressed in the present review. No similar mutations on L-type or T-type channels have been described yet to affect cortisol secretion or to be linked to the development of Cushing syndrome, but several evidences suggest that the function of T channels is also crucial in fasciculata cells. Putative molecular mechanisms and cellular structural organization making T channels a privileged entry for the "steroidogenic calcium" are also discussed.

Intracellular transport of calcium from plasma membrane to mitochondria in adrenal H295R cells: implication for steroidogenesis

Angiotensin II and extracellular potassium stimulate aldosterone production in adrenal glomerulosa cells by mobilizing the calcium messenger system. This response requires calcium influx across the plasma membrane, followed by calcium uptake into the mitochondria. It has been proposed that calcium is transported to the mitochondria via the lumen of the endoplasmic reticulum, acting as a kind of intracellular calcium pipeline. This hypothesis has been tested in the present study by measuring intramitochondrial calcium variations in H295R cells with a new fluorescent calcium probe, ratiometric pericam. Calyculin A, a protein phosphatase inhibitor, induced the formation of a large cortical layer of actin filaments, removing the peripheral endoplasmic reticulum away from the plasma membrane and thereby physically uncoupling the calcium channels from the pipeline. The mitochondrial calcium response to potassium was markedly reduced after calyculin treatment, but that of AngII was unaffected. Under the same conditions, potassium-stimulated pregnenolone and aldosterone production was significantly reduced, whereas the steroidogenic response to AngII remained unchanged. The inhibitory action of calyculin A on the responses to potassium was not mediated by a modification of the calcium channel activity and was not accompanied by a reduction of the cytosolic calcium response. It therefore appears that, in H295R cells, the organization of the actin cytoskeleton at the cell periphery influences the steroidogenic action of potassium, but not the response to angiotensin II. The response to potassium is proposed to be dependent on the endoplasmic reticulum-mediated transfer of calcium entering through plasma membrane calcium channels to the mitochondria.

Angiotensin II-induced inhibition of calcium currents via G(q/11)-protein involving protein kinase C in hamster submandibular ganglion neurons

Angiotensin II (AngII) is one of the most important vasoconstrictive hormones but is also known to act as a neuromodulator and a neurotransmitter in the central and peripheral nervous systems. In a previous study, we have shown that AngII, mediated by AT(1) receptors, inhibits voltage-dependent calcium channel (VDCC) currents (I(Ca)) via G-proteins in submandibular ganglion (SMG) neurons. In this study, we further characterized the signal transduction of AngII-induced inhibition of I(Ca). Application of 1 microM AngII inhibited I(Ca) by 32.1+/-2.7% (mean+/-S.E.M., n=9). Intracellular dialysis of anti-G(q/11) antibodies attenuated these inhibition (8.8+/-1.3%, n=6). In addition, treatment of protein kinase C (PKC) activator and inhibitor also attenuated these inhibition (8.0+/-0.9 and 9.8+/-0.9%, n=6 and 9, respectively). We therefore conclude that AngII inhibits VDCC via G(q/11)-proteins involving in SMG neurons. In addition, such PKC-dependent pathways mediated mainly L-type VDCC inhibition.

Alternatively spliced alpha(1G) (Ca(V)3.1) intracellular loops promote specific T-type Ca(2+) channel gating properties

At least three genes encode T-type calcium channel alpha(1) subunits, and identification of cDNA transcripts provided evidence that molecular diversity of these channels can be further enhanced by alternative splicing mechanisms, especially for the alpha(1G) subunit (Ca(V)3.1). Using whole-cell patch-clamp procedures, we have investigated the electrophysiological properties of five isoforms of the human alpha(1G) subunit that display a distinct III-IV linker, namely, alpha(1G-a), alpha(1G-b), and alpha(1G-bc), as well as a distinct II-III linker, namely, alpha(1G-ae), alpha(1G-be), as expressed in HEK-293 cells. We report that insertion e within the II-III linker specifically modulates inactivation, steady-state kinetics, and modestly recovery from inactivation, whereas alternative splicing within the III-IV linker affects preferentially kinetics and voltage dependence of activation, as well as deactivation and inactivation. By using voltage-clamp protocols mimicking neuronal activities, such as cerebellar train of action potentials and thalamic low-threshold spike, we describe that inactivation properties of alpha(1G-a) and alpha(1G-ae) isoforms can support channel behaviors reminiscent to those described in native neurons. Altogether, these data demonstrate that expression of distinct variants for the T-type alpha(1G) subunit can account for specific low-voltage-activated currents observed in neuronal tissues.

Angiotensin II and calcium channels

Sixty years after its initial discovery, the octapeptide hormone angiotensin II (AngII) has proved to play numerous physiological roles that reach far beyond its initial description as a hypertensive factor. In spite of the host of target tissues that have been identified, only two major receptor subtypes, AT1 and AT2, are currently fully identified. The specificity of the effects of AngII relies upon numerous and complex intracellular signaling pathways that often mobilize calcium ions from intracellular stores or from the extracellular medium. Various types of calcium channels (store- or voltage-operated channels) endowed with distinct functional properties play a crucial role in these processes. The activity of these channels can be modulated by AngII in a positive and/or negative fashion, depending on the cell type under observation. This chapter reviews the main characteristics of AngII receptor subtypes and of the various calcium channels as well as the involvement of the multiple signal transduction mechanisms triggered by the hormone in the cell-specific modulation of the activity of these channels.

Angiotensin II negatively modulates L-type calcium channels through a pertussis toxin-sensitive G protein in adrenal glomerulosa cells

In bovine adrenal glomerulosa cells, angiotensin II and extracellular K+ stimulate aldosterone secretion in a calcium-dependent manner. In these cells, physiological concentrations of extracellular potassium activate both T-type (low threshold) and L-type (high threshold) voltage-operated calcium channels. Paradoxically, the cytosolic calcium response to 9 mM K+ is inhibited by angiotensin II. Because K+-induced calcium changes observed in the cytosol are almost exclusively due to L-type channel activity, we therefore studied the mechanisms of L-type channel regulation by angiotensin II. Using the patch-clamp method in its perforated patch configuration, we observed a marked inhibition (by 63%) of L-type barium currents in response to angiotensin II. This effect of the hormone was completely prevented by losartan, a specific antagonist of the AT1 receptor subtype. Moreover, this inhibition was strongly reduced when the cells were previously treated for 1 night with pertussis toxin. An effect of pertussis toxin was also observed on the modulation by angiotensin II of the K+ (9 mM)-induced cytosolic calcium response in fura-2-loaded cells, as well as on the angiotensin II-induced aldosterone secretion, at both low (3 mM) and high (9 mM) K+ concentrations. Finally, the expression of both Go and Gi proteins in bovine glomerulosa cells was detected by immunoblotting. Altogether, these results strongly suggest that in bovine glomerulosa cells, a pertussis toxin-sensitive G protein is involved in the inhibition of L-type channel activity induced by angiotensin II.