Serotonin stimulates calcium influx in isolated rat adrenal zona glomerulosa cells.

To investigate the role of calcium as a second messenger in serotonin-stimulated aldosterone secretion, radiolabelled calcium influx studies were carried out in purified rat adrenal zona glomerulosa cells using 45CaCl2. The results show that serotonin caused calcium influx within 45 seconds of addition and this continued for up to 105 seconds. Angiotensin II also caused calcium influx; however, the effect was significantly smaller than that of serotonin. Serotonin-stimulated calcium influx could be inhibited by the calcium antagonist verapamil and by methysergide, a selective serotonin receptor type-1/2 antagonist. The data indicate that serotonin directly stimulates calcium uptake in zona glomerulosa cells via calcium channels which are coupled to specific serotonin receptors.     

Background calcium permeable channels in glomerulosa cells from adrenal gland

The cell-attached recording mode of the patch-clamp technique was used to study Ca2+ permeable background currents of glomerulosa cells from rat and bovine adrenal gland. With a pipette filled with 110 mM BaCl2 or 90 mM CaCl2, three different types of unitary currents were detected. The B1 channel demonstrates a nonlinear I-V curve. The conductances are 4 and 7 pS at -40 and -70 mV, respectively. The curve of the opening probability vs. membrane potential is bell shaped with its maximum at -70 mV. The B2 channel has a conductance of 6 pS, while the B3 channel shows a nonlinear I-V relationship with conductances close to 17 and 10 pS at HPs of -60 and -20 mV. The three types of currents are insensitive to dihydropyridines. We suggest that these background currents could be responsible for the basal calcium influx and aldosterone secretion previously observed in nonstimulated glomerulosa cells.     

Evidence for two distinct voltage-gated calcium channel currents in bovine adrenal glomerulosa cells

We analyzed inward Ca2+ currents in single bovine adrenal glomerulosa cell using whole-cell patch clamp techniques. Two types of voltage-gated Ca2+ channel currents were identified. One was a transient (T) type which decayed within 100 ms, characterized by a low threshold voltage (about -70 mv) similar to that seen in rat adrenal glomerulosa cells (Matsunaga, H. et al. (1987) Pflügers Arch. 408, 351-355.) Another was a long-lasting (L) type which shows a more positive threshold potential. The present results suggest that while T type Ca2+ channels may explain initial calcium influx in response to an elevation in extracellular K+, L type Ca2+ channels may allow sustained calcium influx which is necessary for sustained aldosterone secretion.     

Stimulus-secretion coupling in angiotensin-stimulated adrenal glomerulosa cells

Adrenal glomerulosa cell is a suitable model for a comparative study of signal transducing mechanisms since its secretory activity is regulated by at least three different mechanisms: the adenylate cyclase-cAMP system (for ACTH), the voltage-dependent Ca2+ channel (for K+ and perhaps for angiotensin II) and the inositol 1,4,5-trisphosphate-Ca2+ system (for angiotensin II and vasopressin). The role of inositol phosphates, extracellular Ca2+ and protein kinase C in the induction and sustaining of aldosterone production by cells exposed to angiotensin II is critically reviewed.     

An oxidized metabolite of linoleic acid increases intracellular calcium in rat adrenal glomerulosa cells

EKODE, an epoxy-keto derivative of linoleic acid, was previously shown to stimulate aldosterone secretion in rat adrenal glomerulosa cells. In the present study, we investigated the effect of exogenous EKODE on cytosolic [Ca(2+)] increase and aimed to elucidate the mechanism involved in this process. Through the use of the fluorescent Ca(2+)-sensitive dye Fluo-4, EKODE was shown to rapidly increase intracellular [Ca(2+)] ([Ca(2+)](i)) along a bell-shaped dose-response relationship with a maximum peak at 5 microM. Experiments performed in the presence or absence of Ca(2+) revealed that this increase in [Ca(2+)](i) originated exclusively from intracellular pools. EKODE-induced [Ca(2+)](i) increase was blunted by prior application of angiotensin II, Xestospongin C, and cyclopiazonic acid, indicating that inositol trisphosphate (InsP(3))-sensitive Ca(2+) stores can be mobilized by EKODE despite the absence of InsP(3) production. Accordingly, EKODE response was not sensitive to the phospholipase C inhibitor U-73122. EKODE mobilized a Ca(2+) store included in the thapsigargin (TG)-sensitive stores, although the interaction between EKODE and TG appears complex, since EKODE added at the plateau response of TG induced a rapid drop in [Ca(2+)](i). 9-oxo-octadecadienoic acid, another oxidized derivative of linoleic acid, also increases [Ca(2+)](i), with a dose-response curve similar to EKODE. However, arachidonic and linoleic acids at 10 microM failed to increase [Ca(2+)](i) but did reduce the amplitude of the response to EKODE. It is concluded that EKODE mobilizes Ca(2+) from an InsP(3)-sensitive store and that this [Ca(2+)](i) increase is responsible for aldosterone secretion by glomerulosa cells. Similar bell-shaped dose-response curves for aldosterone and [Ca(2+)](i) increases reinforce this hypothesis.     

Control of phosphatidylinositol turnover in adrenal glomerulosa cells

The purpose of the present experiments was to compare the effects on phosphatidylinositol metabolism of agents stimulating aldosterone secretion. Glomerulosa cells, isolated from rat adrenals, were incubated in the presence of one of the following stimuli: angiotensin II, elevated potassium concentration, corticotropin, dibutyryl cyclic AMP and prostaglandin E2. Of all these substances, only angiotensin II stimulated the incorporation of [32P]phosphate into phosphatidylinositol. The effect was already detected 2.5 min and was still maintained 60 min after the onset of stimulation. A slight enhancement of the incorporation into other phospholipids was observed in the first minutes of stimulation. Cycloheximide abolished the effect of angiotensin II on aldosterone production, but not on phosphatidylinositol synthesis. In cells prelabelled with [32P]phosphate, radioactivity in phosphatidylinositol relative to that in other phospholipids decreased in response to angiotensin II within 5 min. This indicates that angiotensin II induces a specific breakdown of phosphatidylinositol. Corticotropin failed to enhance the incorporation of [32P]phosphate into phosphatidylinositol and other phospholipids in isolated fasciculate-reticularis cells. The results suggests that although both angiotensin II and potassium are presumed to act through changes in calcium metabolism, angiotensin alone generates the calcium signal by increased phosphatidylinositol turnover.     

Intramitochondrial bodies in sheep adrenal zona glomerulosa cells

Summary Relatively large, mostly rounded, very electron dense intramitochondrial bodies in adrenal zona glomerulosa cells of sheep are described and their nature and connection to protein in the mitochondria discussed. The so called azocarmine granules seen in the light microscope may be identical with the intramitochondrial bodies in the zona glomerulosa cells.     

Role of voltage-gated calcium channels in potassium-stimulated aldosterone secretion from rat adrenal zona glomerulosa cells

The mineralocorticoid aldosterone plays an important role in the regulation of plasma electrolyte homeostasis. Exposure of acutely isolated rat adrenal zona glomerulosa cells to elevated K(+) activates voltage-gated calcium channels and initiates a calcium-dependent increase in aldosterone synthesis. We developed a novel 96-well format aldosterone secretion assay to rapidly evaluate the effect of known T- and L-type calcium channel antagonists on K(+)-stimulated aldosterone secretion and better define the role of voltage-gated calcium channels in this process. Reported T-type antagonists, mibefradil and Ni(2+), and selected L-type antagonist dihydropyridines, inhibited K(+)-stimulated aldosterone synthesis. Dihydropyridine-mediated inhibition occurred at concentrations which had no effect on rat alpha1H T-type Ca(2+) currents. In contrast, below 10 microM, the L-type antagonists verapamil and diltiazem showed only minimal inhibitory effects. To examine the selectivity of the calcium channel antagonist-mediated inhibition, we established an aldosterone secretion assay in which 8Br-cAMP stimulates aldosterone secretion independent of extracellular calcium. Mibefradil remained inhibitory in this assay, while the dihydropyridines had only limited effects. Taken together, these data demonstrate a role for the L-type calcium channel in K(+)-stimulated aldosterone secretion. Further, they confirm the need for selective T-type calcium channel antagonists to better address the role of T-type channels in K(+)-stimulated aldosterone secretion.     

Characterization of the 1,25-dihydroxycholecalciferol-stimulated calcium influx pathway in CaCo-2 cells

The present studies were conducted to investigate the mechanisms underlying the 1,25-dihydroxycholecalciferol (1,25(OH)₂D₃)-induced increase in intracellular Ca²⁺ ([Ca²⁺] _i ) in individual CaCo-2 cells. In the presence of 2mm Ca²⁺, 1,25(OH)₂D₃-induced a rapid transient rise in [Ca²⁺] _i in Fura-2-loaded cells in a concentration-dependent manner, which decreased, but did not return to baseline levels. In Ca²⁺-free buffer, this hormone still induced a transient rise in [Ca²⁺] _i , although of lower magnitude, but [Ca²⁺] _i then subsequently fell to baseline. In addition, 1,25(OH)₂D₃ also rapidly induced⁴⁵Ca uptake by these cells, indicating that the sustained rise in [Ca²⁺] _i was due to Ca²⁺ entry. In Mn²⁺-containing solutions, 1,25(OH)₂D₃ increased the rate of Mn²⁺ influx which was temporally preceded by an increase in [Ca²⁺] _i . The sustained rise in [Ca²⁺] _i was inhibited in the presence of external La³⁺ (0.5mm). 1,25(OH)₂D₃ did not increase Ba²⁺ entry into the cells. Moreover, neither high external K⁺ (75mm), nor the addition of Bay K 8644 (1 μm), an L-type, voltage-dependent Ca²⁺ channel agonist, alone or in combination, were found to increase [Ca²⁺] _i , 1,25(OH)₂D₃ did, however, increase intracellular Na⁺ in the absence, but not in the presence of 2mm Ca²⁺, as assessed by the sodium-sensitive dye, sodium-binding benzofuran isophthalate. These data, therefore, indicate that CaCo-2 cells do not express L-type, voltage-dependent Ca²⁺ channels. 1,25(OH)₂D₃ does appear to activate a La³⁺-inhibitable, cation influx pathway in CaCo-2 cells.     

Characteristics of angiotensin II-, K+- and ACTH-induced calcium influx in adrenal glomerulosa cells. Evidence that angiotensin II, K+, and ACTH may open a common calcium channel

The characteristics of angiotensin II-, K+-, and adrenocorticotropin (ACTH)-induced calcium influx were studied in isolated adrenal glomerulosa cells. Basal calcium influx rate is 0.64 +/- 0.09 nmol/min/mg of protein. Addition of angiotensin II (1 nM) causes a rapid 230% increase in calcium influx rate. This angiotensin II-induced calcium influx is sustained and is rapidly reversed by angiotensin II antagonist, [Sar1,Ala8]angiotensin II. Addition of either K+ or ACTH (1 nM) causes a 340 or 160% increase, respectively, in the rate of calcium influx. The effect of either angiotensin II, K+, or ACTH on calcium influx is dependent on extracellular calcium. The apparent Km for calcium is 0.46, 0.35, and 0.32 mM, respectively. When the extracellular concentration of K+ is 2 mM, neither angiotensin II nor ACTH stimulates calcium influx. Conversely, when extracellular K+ is increased to 6 mM, both angiotensin II and ACTH cause a greater stimulation of calcium influx than at 4 mM K+. When extracellular K+ is increased to 10 mM, calcium influx is 360% of the basal influx seen at 4 mM K+, and neither angiotensin II nor ACTH further stimulates the influx rate. Nitrendipine (1 microM) blocks both angiotensin II- and K+-induced calcium influx completely. In contrast, 10 microM nitrendipine does not completely block ACTH-induced calcium influx. The calcium channel agonist, BAY K 8644, also stimulates calcium influx; 10 nM BAY K 8644 leads to a rate of calcium influx which is 185% of basal. This BAY K 8644-induced increase in calcium influx and that caused by either angiotensin II or ACTH are additive. In contrast, BAY K 8644 has more than an additive effect on the calcium influx when paired with 6 mM K+. These results suggest that angiotensin II, K+, and ACTH stimulate calcium influx via a common calcium channel but act by different mechanisms to alter its function.     

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.     

Dual effects of dopamine in rat adrenal glomerulosa cells

The effects of dopamine (DA) on cAMP production and aldosterone secretion were compared in freshly isolated cells and in primary cultures of rat adrenal glomerulosa cells. Under isolated conditions, glomerulosa cells exhibited dopamine receptors from DA-1 and DA-2 subclass, whereas in cultured conditions, where cells are very sensitive to their known stimuli, cells only exhibited dopamine receptors from the DA-1 subclass. Moreover, unlike freshly isolated cells, dopamine stimulated both cAMP production and aldosterone secretion in 3-day cultured preparations. These effects were receptor specific since they were completely suppressed by Scherring 23390 (a specific DA-1 antagonist) and were unaffected by a beta-adrenergic antagonist. As in vivo rat adrenal cortex contains DA, we discuss a possible involvement of this neurotransmitter in the regulation of aldosterone secretion.     

Angiotensin II and FCCP mobilizes calcium from different intracellular pools in adrenal glomerulosa cells; analysis of calcium fluxes

The aim of the present study was to examine the effect of angiotensin II on the different pools of exchangeable Ca2+ in isolated rat adrenal glomerulosa cells. On the basis of steady state analysis of 45Ca exchange curves at least three kinetically distinct Ca2+ compartments are present in these cells. The most rapidly exchangeable compartment was regarded as Ca2+ loosely bound to the glycocalyx and the other compartments were considered to be intracellular Ca2+ pools. The effect of angiotensin II on different intracellular compartments was examined by adding the hormone at different phases of Ca2+ washout. Angiotensin increased the rate of 45Ca efflux within 1.5 min when added at the beginning of the washout. This effect, however, could not be detected when the hormone was added at the 30th min of washout, indicating that at least one hormone sensitive pool had lost most of its radioactivity by this time. In contrast to angiotensin II, the mitochondrial uncoupler FCCP mobilized almost the same quantity of 45Ca irrespective of the time of its addition during the washout. This latter finding suggests that this presumably mitochondrial Ca2+ pool has a slow rate of exchange and thus differs from the pool initially mobilized by angiotensin II. The initial Ca2+ mobilizing effect of angiotensin II was also observed in a Ca2+-free media which contained EGTA, indicating that this effect is not triggered by increased Ca2+ influx. In the present study we demonstrate in the intact glomerulosa cell that angiotensin II mobilizes Ca2+ from an intracellular Ca2+ store which appears to be distinct from the FCCP-sensitive store.     

Effects of low extracellular sodium on cytosolic ionized calcium. Na+-Ca2+ exchange as a major calcium influx pathway in kidney cells

The effects of extracellular Na+ (Na+o) on cytosolic ionized calcium (Ca2+i) and on calcium and sodium fluxes were measured in monkey kidney cells (LLC-MK2). Ca2+i was measured with aequorin and the ion fluxes with 45Ca and 22Na. Na+-free media rapidly increased Ca2+i from 60 to a maximum of about 700 nM in 2-3 min. After the peak, Ca2+i declined and reached a plateau of about twice the resting Ca2+i. The peak Ca2+i was inversely proportional to Na+o and directly proportional to the extracellular calcium concentration (Ca2+o). On the other hand, a pH of 6.8 reduced and Ca2+o substitution with Sr2+ completely blocked the Ca2+i response to low Na+o. A Na+-free medium stimulated calcium efflux from the cells 4-5-fold, a response which was abolished in the absence of extracellular Ca2+. Na+-free media also stimulated calcium influx and sodium efflux. The cell calcium content, however, was not increased. These results indicate that removal of extracellular Na+ increases Ca2+i by stimulating calcium influx and not by inhibiting calcium efflux; the increased calcium influx takes place on the Na+-Ca2+ antiporter operating in the reverse mode in exchange for sodium efflux. The increased calcium efflux occurs as a consequence of the rise in Ca2+i and presumably takes place on the (Ca2+-Mg2+) ATPase-dependent calcium pump.     

Dopamine in rat adrenal glomerulosa

There is increasing evidence that dopamine (DA) inhibits aldosterone production, but the source of DA for this dopaminergic influence is not known. In the present study we examined the adrenal's zona glomerulosa for the presence of DA. Rats maintained on an intake of regular food were killed by decapitation and the adrenal capsule (containing zona glomerulosa) and the remainder of the gland (containing both cortex and medulla) were examined for their content of DA and also for norepinephrine (NE) and epinephrine (E). DA was found in adrenal glomerulosa in substantial quantity, 1.92 +/- 0.17 (SEM) ng/mg wet weight, representing an approximate concentration of DA of 1-100 microM. DA in adrenal capsule represented 12.2% of the total adrenal content of DA. NE and E were also present in glomerulosa, 3.46 +/- 0.32 and 18.7 +/- 2.1 ng/mg respectively, but, unlike DA, about 98% of the total adrenal content of NE and E was contained in adrenal medulla. The NE/E ratio in capsule and medulla were similar, although slightly higher in adrenal medulla, suggesting that the medulla is the source of the NE and E found in glomerulosa. On the other hand, the DA/E ratio was several-fold higher in glomerulosa than medulla--suggesting that glomerulosa DA was derived at least partially from a source other than adrenal medulla. We also found that short-term culturing of the adrenal reduced DA levels to 1/3 that observed in fresh tissue. This could explain in part why cultured glomerulosa has been shown to be more responsive to administered stimuli. In summary, the findings indicate a significant concentration of DA in adrenal glomerulosa, and suggest that the effects of DA on aldosterone production are mediated locally within the adrenal.     

Control of calcium homeostasis by angiotensin II in adrenal glomerulosa cells through activation of p38 MAPK

Angiotensin II-induced activation of aldosterone secretion in adrenal glomerulosa cells is mediated by an increase of intracellular calcium. We describe here a new Ca2+-regulatory pathway involving the inhibition by angiotensin II of calcium extrusion through the Na+/Ca2+ exchanger. Caffeine reduced both the angiotensin II-induced calcium signal and aldosterone production in bovine glomerulosa cells. These effects were independent of cAMP or calcium release from intracellular stores. The calcium response to angiotensin II was more sensitive to caffeine than the response to potassium, suggesting that the drug interacts with a pathway specifically elicited by the hormone. In calcium-free medium, calcium returned more rapidly to basal levels after angiotensin II stimulation in the presence of caffeine. Thapsigargin had no effect on these kinetics, but diltiazem, which inhibits the Na+/Ca2+ exchanger, markedly reduced the rate of calcium decrease and abolished caffeine action. The involvement of this exchanger was supported by the effect of cell depolarization and of a reduction of extracellular sodium on the rate of calcium extrusion. We also determined the mechanism of angiotensin II action on the exchanger. Phorbol esters reduced the rate of calcium extrusion, which was increased by baicalein, an inhibitor of lipoxygenases, and by SB 203580, an inhibitor of the p38 MAPK. Finally, we showed that angiotensin II acutely activates, in a caffeine-sensitive manner, p38 MAPK in glomerulosa cells. In conclusion, in bovine glomerulosa cells, the Na+/Ca2+ exchanger plays a crucial role in extruding calcium, and, by reducing its activity, angiotensin II influences the amplitude of the calcium signal. The hormone exerts its action on the exchanger through a caffeine-sensitive pathway involving the p38 MAPK and lipoxygenase products.     

Internalization of atrial natriuretic peptide by adrenal glomerulosa cells

Internalization of 125I-labelled atrial natriuretic peptide ([ 125I]ANP) by rat adrenal glomerulosa cells in vivo was investigated by means of an ultrastructural autoradiographic approach. One to 30 min after IV injection of [125I]ANP, silver grains were found, at the light microscope level, over all glomerulosa cells; coinjection of 20 micrograms of unlabelled ANP inhibited this binding by 64%. At the electron microscope level, the time-course study indicated maximal silver grain densities in plasma membranes 1 min after IV injection; grains were detected in mitochondria (external membranes and matrix) 2 min after injection, with maximal labelling at 15 min. The cytoplasmic matrix was labelled only 30 min after injection. During the time-course, labelling of nuclei, Golgi apparatus, and lysosomes was minimal. The data suggest that after binding to plasma membranes ANP is rapidly internalized and distributed within glomerulosa cells. The association of radioactivity with mitochondria suggests that ANP may have intracellular sites of action complementary to those on plasma membranes.     

Time-dependent restoration of the trigger pool of calcium after termination of angiotensin II action in adrenal glomerulosa cells

In adrenal glomerulosa cells, angiotensin II causes an immediate release of calcium from an intracellular trigger pool (Kojima, I., Kojima, K., and Rasmussen, H. (1985) Am. J. Physiol. 247, E36-E43). The present study was conducted to determine how the trigger pool of calcium is restored after cessation of the agonist action. Upon termination of angiotensin II action, calcium influx rate decreased immediately while total cell calcium increased rapidly. The increase in total cell calcium is not affected by 1 microM nitrendipine, which blocks angiotensin II-stimulated calcium influx without inhibiting basal influx of calcium. In contrast, total cell calcium did not increase in medium containing 1 microM calcium, in which basal calcium influx is negligible. A rapid increase in total cell calcium after an addition of the antagonist was not accompanied by changes in cytoplasmic free calcium concentration. A second stimulation of cells with either angiotensin II or carbachol did not cause calcium release when the interval of two stimulations was shorter than 20 min. The longer the interval, the greater the magnitude of calcium release in response to the second stimulator. The maximum response was obtained when the interval was 40 min or more. When exogenous arachidonic acid, which mobilized calcium by acting directly on the inositol trisphosphate-sensitive pool, was employed as a second stimulator, the magnitude of the decrease in total cell calcium was also dependent on the interval. These results suggest that, upon termination of angiotensin II action, calcium is rapidly accumulated first in an intracellular pool which is insensitive to either inositol 1,4,5-trisphosphate or arachidonic acid and that the trigger pool is restored gradually thereafter.     

Progesterone metabolites rapidly stimulate calcium influx in human platelets by a src-dependent pathway

The effects of several steroids and their metabolites were examined for their ability to rapidly alter intracellular free calcium ([Ca(2+)](i)) in the anucleate human platelet. Earlier studies suggested that steroids had direct and rapid non-genomic effects to alter platelet physiology. The rationale for performing this study was to investigate the signal transduction events being activated by steroids. Super-physiologic concentrations (1.0-10.0microM) of beta-estradiol and several estradiol metabolites and analogs potentiated (approximately twofold) the action of thrombin to elevate [Ca(2+)](i) in platelets, whereas 10.0microM progesterone inhibited the action of thrombin by 10-15%. Progesterone and beta-estradiol by themselves did not affect [Ca(2+)](i). Progesterone metabolites can achieve high blood concentrations. Some progesterone metabolites, particularly those in the beta-conformation, were potent stimulators of Ca(2+) influx and intracellular Ca(2+) mobilization in platelets. They activated phospholipase C because their ability to increase [Ca(2+)](i) was inhibited by the phospholipase C inhibitor U-73122. The ability of pregnanediol and collagen to increase [Ca(2+)](i) was inhibited by the src tyrosine kinase inhibitor PP1, whereas the actions of thrombin and thapsigargin to increase [Ca(2+)](i) were not affected by PP1. The effects of progesterone metabolites to increase [Ca(2+)](i) were observed with concentrations as low as 0.1microM. Pregnanolone synergized with thrombin to increase [Ca(2+)](i). It is hypothesized that human platelets possess receptors for progesterone metabolites. These receptors when stimulated will activate platelets by causing a rapid increase in [Ca(2+)](i). Pregnanolone, isopregnanediol and pregnanediol were the most effective stimulators of this newly identified src-dependent signal transduction system in platelets. Progesterone metabolites may regulate platelet aggregation and hence thrombosis in vivo.     

Angiotensin II receptors and mechanisms of action in adrenal glomerulosa cells

The plasma-membrane receptors, coupling mechanisms, and effector enzymes that mediate target-cell activation by angiotensin II (AII) have been characterized in rat and bovine adrenal glomerulosa cells. The AII holoreceptor is a glycoprotein of Mr approximately 125,000 under non-denaturing conditions. Photoaffinity labeling of AII receptors with azido-AII derivatives has shown size heterogeneity among the AII binding sites between species and target tissues, with Mr values of 55,000 to 79,000. Such variations in molecular size probably reflect differences in carbohydrate content of the individual receptor sites. The adrenal AII receptor, like that in other tissues, is coupled to the inhibitory guanine nucleotide inhibitory protein (Ni). However, studies with pertussis toxin have shown that stimulation of aldosterone production by AII is not mediated by Ni but by a pertussis-insensitive nucleotide regulatory protein of unidentified nature. Although Ni is not involved in the stimulatory action of AII on steroidogenesis, it does mediate the inhibitory effects of high concentrations of AII upon aldosterone production. The actions of AII on adrenal cortical function are thus regulated by at least two guanine nucleotide regulatory proteins that are selectively activated by increasing AII concentrations. The principal effector enzyme in AII action is phospholipase C, which is rapidly stimulated in rat and bovine glomerulosa after AII receptor activation. AII-induced breakdown of phosphatidylinositol bisphosphate (PIP2) and phosphatidylinositol phosphate (PIP) leads to formation of inositol 1,4,5-trisphosphate (IP3) and inositol 1,4-bisphosphate (IP2). These are metabolized predominantly to inositol-4-monophosphate, which serves as a marker of polyphosphoinositide breakdown, whereas inositol-1-phosphate is largely derived from phosphatidylinositol hydrolysis. The AII-stimulated glomerulosa cell also produces inositol 1,3,4-trisphosphate, a biologically inactive IP3 isomer formed from Ins-1,4,5-trisphosphate via inositol tetrakisphosphate (IP4) during ligand activation in several calcium-dependent target cells. The Ins-1,4,5-P3 formed during AII action binds with high affinity to specific intracellular receptors that have been characterized in the bovine adrenal gland and other AII target tissues, and may represent the sites through which IP3 causes calcium mobilization during the initiation of cellular responses.