S. Tomlinson

Calmodulin regulation of adenylate cyclase activity

Calmodulin-dependent stimulation of adenylate cyclase was initially thought to be a unique feature of neural tissues. In recent years evidence to the contrary has accumulated, calmodulin-dependent stimulation of adenylate cyclase now being demonstrated in a wide range of structurally unrelated tissues and species. Demonstration of the existence of calmodulin-dependent adenylate cyclase has in nearly all instances required the removal of endogenous calmodulin. It is not yet clear whether calmodulin-dependent and calmodulin-independent forms of the enzyme exist and whether some tissues (such as heart) lack a calmodulin-dependent adenylate cyclase. The presence of calmodulin appears largely responsible for the ability of the adenylate cyclase enzyme to be stimulated by submicromolar concentrations of calcium; it may not be relevant to the inhibition of the enzyme which occurs at higher concentrations of calcium. The physical relationship of calmodulin to the plasma membrane bound enzyme (or to the soluble forms of the enzyme) is not known nor is the mechanism of adenylate cyclase activation by calmodulin clear; current data suggest some involvement with both the N and C units of the enzyme. Finally, it is possible that in vivo calcium contributes to the duration of the hormone stimulated cyclic AMP signal. Thus current in vitro data suggest that optimal hormonal activation of calmodulin-dependent adenylate cyclase occurs at very low intracellular calcium concentrations, comparable to those found in the resting cell; conversely the enzyme is inhibited as intracellular calcium increases, following for example agonist stimulation of the cell. These higher calcium concentrations would then activate calmodulin-dependent phosphodiesterase. Such differential effects of calcium on adenylate cyclase and phosphodiesterase would ultimately restrict the duration of the hormone-induced cyclic AMP signal.

Calcium, calmodulin, and the production of prostacyclin by cultured vascular endothelial cells

The production of prostacyclin (PGI2) by cultured porcine aortic endothelial cells, in response to serum and the calcium ionophore A23187, was inhibited by TMB-8, an antagonist of intracellular calcium mobilization. The calcium-channel blocker methoxyverapamil (D600) inhibited serum-induced PGI2 production in but had little effect on A23187-induced PGI2 production. Calmodulin activity was detected in endothelial-cell Jysates and was inhibited by the calmodulin antagonist W7, which also inhibited PGI2 production in response to both agonists. Calcium and calmodulin appear to play an important role in mediating PGI2 production by the vascular endothelium.

Calmodulin in human serum and the specific release of calmodulin from calmodulin-rich platelets

Calmodulin(CaM)-like activity was detected in human serum and foetal calf serum, with an activity i0 times more than that detectable in plasma. Serum CaM was largely accounted for by release from human platelets as confirmed by both radioimmunoassay and sodium-dodecyl-sulphate/polyacrylamide-gel electrophoresis of CaM partially purified from platelet releasate obtained in response to thrombin. Lactate dehydrogenase release was unaffected by thrombin. Platelet CaM content was very variable (1.3 to 11.3 pg/mg protein; n=15). It is suggested that intact platelets are rich in CaM and that release of CaM during preparation explains the variation in CaM content reported here and in the literature.

Calmodulin and insulin secretion

ConclusionIt is clear that calcium ions are of considerable importance as a second messenger in insulin secretion. There is increasing evidence that calmodulin, a ubiquitous intracellular regulatory protein that mediates calcium-dependent processes, has a fundamental role in stimulus-secretion coupling. Calmodulin is present in the B cell and the secretion of insulin is inhibited by phenothiazines which bind to and inhibit the action of calmodulin. The evidence strongly suggests that phenothiazines influence insulin secretion by their effect on calmodulin which probably mediates calcium-dependent insulin release. It seems likely that calmodulin acts at several points in stimulus-secretion coupling, influencing cyclic nucleotide metabolism, protein phosphorylation and exocytosis.The discovery of calmodulin and the increasing clarification of its roles in cellular metabolism represent major steps towards our understanding of the mechanisms which influence the secretion of insulin.

A role for intracellular calcium and calmodulin in the release of triiodothyronine from human thyroid-cell monolayer cultures

Human thyroid cells in monolayer responded to acute stimulation by TSH with an increase in the secretion of T3. This process appeared to be dependent on a rise in the cytosolic calcium concentration since the antagonist of intraceliular calcium mobilization, TMB-8, was found to inhibit the release of T3 in response to TSH. The importance of intracellular calcium was further shown using the agent veratridine which increases the free calcium level within cells; veratridine potentiated the stimulation of T3 secretion by TSH and itself stimulated the release of T3 to a level higher than that seen in the presence of TSH alone. The calcium ionophore A23197 produced a biphasic effect on T3 secretion from human thyroid monolayers; at low concentrations, A23187 caused a decrease in both unstimulated and TSH-stimulated T3 secretion but above a concentration of 1 μM, T3 secretion was increased. The calmodulin antagonist W7 was found to inhibit T3 release in response to TSH, indicating a role for calmodulin in mediating the effects of intracellular calcium on T3 secretion.

Evidence that calmodulin may be involved in phytohaemagglutinin-stimulated lymphocyte division

Phytohaemagglutinin-stimulated and non-stimulated incorporation of [3H]thymidine into human peripheral blood lymphocytes is inhibited by the calcium antagonist PY 108–068 and by the calmodulin antagonists trifluo-perazine andN-(6-aminohexyl)-5-chloro-l-naphthalene sulphonamide (W7). It is argued that calmodulin may be involved in both non-stimulated [3H]thymidine uptake in lymphocytes and also in the lymphocyte response to phytohaemagglutinin.

The possible role of calmodulin in the inhibition of prolactin secretion by dopaminergic antagonists

Several previous reports have indicated that a number of dopaminergic antagonists paradoxically inhibit prolactin secretion at micromolar concentrations. It is well known that some of these drugs, including pimozide and the phenothiazines, are inhibitors of calmodulin activity. Here we report that micromolar concentrations of several dopaminergic antagonists inhibit prolactin secretion from isolated rat anterior pituitary cells and calmodulin activity (calmodulin-activated cyclic GMP phosphodiesterase). Inhibition of calmodulin activity may thus, at least partially, explain the inhibitory effect of these drugs on prolactin secretion.

Calmodulin levels in psoriasis—the effect of treatment

Calmodulin (CaM) is an intracellular calcium binding protein which appears to have an essential modulatory role in a variety of intracellular processes. Levels of CaM in the epidermis in psoriasis have been shown to be elevated in both lesional and non‐lesional skin (Van de Kerkhof & Van Erp, 1983; Tucker et al., 1984).