Does calmodulin play a role in the regulation of cardiac sarcolemmal adenylate cyclase activity?

Suppression of cAMP by phosphoinositol/Ca2+ pathway in the cardiac kappa-opioid receptor

To determine whether the phosphoinositol/Ca2+ pathway interacts with the adenylate cyclase/adenosine 3',5'-cyclic monophosphate (cAMP) pathway in the cardiac kappa-receptor, the effects of U-50488, a specific kappa-receptor agonist, on the intracellular Ca2+ concentration ([Ca2+]i) and forskolin-induced accumulation of cAMP in rat ventricular myocytes were determined after interference of the phosphoinositol/Ca2+ pathway. U-50488 suppressed the forskolin-induced accumulation of cAMP and elevated [Ca2+]i, which were blocked by norbinaltorphimine, a specific kappa-receptor antagonist, and pertussis toxin. The effects of U-50488 were qualitatively similar to those of A-23187, a Ca2+ ionophore, but opposite to those of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM), a [Ca2+]i chelator. Abolition of U-50488-induced elevation of [Ca2+]i by BAPTA-AM also abolished the effect of U-50488 on forskolin-induced accumulation of cAMP. Inhibition of the phospholipase C by specific inhibitors, U-73122 and neomycin, abolished the effects of U-50488 on both [Ca2+]i and forskolin-induced accumulation of cAMP. The results showed for the first time that kappa-receptor stimulation may suppress cAMP accumulation via activation of the phosphoinositol/Ca2+ pathway in the rat heart.

The inhibition of human duodenal adenylate cyclase activity by Ca2+ and the effects of EGTA

This study demonstrates that the inhibition of adenylate cyclase activity by Ca2+ is enhanced in the presence of increasing [EGTA] (0, 0.3, 1, 2.5 mM) by 2 orders of magnitude. It has been established that this effect is not because of poor Ca2+ buffering by low [EGTA] or high Ca2+ binding by the membrane preparation. It is present irrespective of stimulus. We suggest the enhanced sensitivity of adenylate cyclase to Ca2+ induced by EGTA is caused by the Ca-EGTA complex being a more inhibitory species than Ca2+. Thus consideration of the effects of the Ca-EGTA complex should be made when interpreting the results from experiments involving Ca2+ and EGTA.

The type III calcium/calmodulin-sensitive adenylyl cyclase is not specific to olfactory sensory neurons

A cDNA clone for type III adenylyl cyclase was originally isolated from a rat olfactory cDNA library and Northern analysis using total RNA suggested that the expression of the type III mRNA may be limited to the olfactory epithelium (Bakalyar and Reed, Science, 250 (1990) 1403-1406). In this study, the distribution of type III adenylyl cyclase mRNA in a number of bovine tissues and cultured cells was examined by Northern analysis using poly(A)+ RNA. Type III adenylyl cyclase mRNA was expressed in brain, spinal cord, adrenal medulla, adrenal cortex, heart atrium, aorta, lung, retina, 293 cells and PC-12 cells. Furthermore, the Ca2+ sensitivity of adenylyl cyclase activity in 293 cells indicated the presence of type III adenylyl cyclase. These data indicate that expression of the type III adenylyl cyclase is not limited to olfactory tissues, and that this enzyme probably has a number of physiological functions in addition to olfactory signal transduction.

Calmodulin independence of human duodenal adenylate cyclase

The calmodulin and calcium dependence of human adenylate cyclase from the second part of the duodenum was assessed in washed particulate preparations of biopsy specimens by investigating (a) the concentration dependent effects of free [Ca2+] on enzyme activity, (b) the effects of exogenous calmodulin on enzyme activity in ethylene glycol bis (b-aminoethyl ether)N,N'-tetra-acetic acid (EGTA) washed particulate preparations, and (c) the effects of calmodulin antagonists on enzyme activity. Both basal (IC50 = 193.75 (57.5) nmol/l (mean (SEM)) and NaF stimulated (IC50 = 188.0 (44.0) nmol/l) adenylate cyclase activity was strongly inhibited by free [Ca2+] greater than 90 nmol/l. Free [Ca2+] less than 90 nmol/l had no effect on adenylate cyclase activity. NaF stimulated adenylate cyclase activity was inhibited by 50% at 2.5 mmol/l EGTA. This inhibition could not be reversed by free Ca2+. The addition of exogenous calmodulin to EGTA (5 mmol/l) washed particulate preparations failed to stimulate adenylate cyclase activity. Trifluoperazine and N-(8-aminohexyl)-5-IODO-1-naphthalene-sulphonamide (IODO 8) did not significantly inhibit basal and NaF stimulated adenylate cyclase activity when measured at concentrations of up to 100 mumol/l. These results suggest that human duodenal adenylate cyclase activity is calmodulin independent but is affected by changes in free [Ca2+].

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.