Protein kinase Cdelta mediates ethanol-induced up-regulation of L-type calcium channels

Brief ethanol exposure inhibits L-type, voltage-gated calcium channels in neural cells, whereas chronic exposure increases the number of functional channels. In PC12 cells, this adaptive response is mediated by protein kinase C (PKC), but the PKC isozyme responsible is unknown. Since chronic ethanol exposure increases expression of PKCdelta and PKCepsilon, we investigated the role these isozymes play in up-regulation of L-type channels by ethanol. Incubation with the PKC inhibitor GF 109203X or expression of a PKCdelta fragment that inhibits phorbol ester-induced PKCdelta translocation largely prevented ethanol-induced increases in dihydropyridine binding and K+-stimulated 45Ca2+ uptake. A corresponding PKCepsilon fragment had no effect on this response. These findings indicate that PKCdelta mediates up-regulation of L-type channels by ethanol. Remaining responses to ethanol in cells expressing the PKCdelta fragment were not inhibited by GF 109203X, indicating that PKCdelta-independent mechanisms also contribute. PKCdelta overexpression increased binding sites for dihydropyridine and L-channel antagonists, but did not increase K+-stimulated 45Ca2+ uptake, possibly because of homeostatic responses that maintain base-line levels of channel function. Since L-type channels modulate drinking behavior and contribute to neuronal hyperexcitability during alcohol withdrawal, these findings suggest an important role for PKCdelta in alcohol consumption and dependence.

Stimulation of cyclic AMP accumulation and phosphoinositide hydrolysis by M3 muscarinic receptors in the rat peripheral lung

The effects of oxotremorine-M (oxo-M), a muscarinic agonist, on cyclic AMP (cAMP) accumulation in slices of the rat peripheral lung were investigated. Oxo-M stimulated cAMP accumulation in a concentration-dependent manner with an EC50 value of 4.2 microM and a maximal effect of 2.4 +/- 0.39-fold over basal. In the presence of forskolin (25 microM), the maximal effect of oxo-M was increased to 14.1 +/- 4.0-fold over basal. Forskolin alone caused a 5.9 +/- 2.2-fold increase in cAMP relative to basal; therefore, the combination of both drugs was more than additive. The effects of oxo-M on cAMP accumulation were unaffected by tetrodotoxin, indicating that the action of oxo-M was not mediated by neuronal release of neurotransmitters. Oxo-M had a small inhibitory effect on cAMP in a homogenate preparation, indicating that the stimulatory response to oxo-M in slices of the lung is not due to direct stimulation of adenylyl cyclase. Characterization of the oxo-M potentiation of forskolin-stimulated cAMP accumulation using different muscarinic antagonists yielded calculated pKB values that agreed with binding affinities for the M3 subtype. Oxo-M elicited phosphoinositide hydrolysis in the lung, and the nature of the antagonism of this response was also consistent with that expected for an M3-mediated response. cAMP accumulation in the presence of oxo-M (100 microM), forskolin (12 microM), or both drugs combined was inhibited by indomethacin (1 microM). These results demonstrate that the M3 receptor stimulates cAMP accumulation and phosphoinositide hydrolysis in the rat peripheral lung, and the mechanism for cAMP stimulation may involve arachidonic acid metabolites.

Inhibition of muscarinic stimulated phosphoinositide hydrolysis in the rat parotid gland by cAMP

The ability of agents that increase or mimic cAMP to affect muscarinic receptor mediated phosphoinositide hydrolysis was investigated in the rat parotid gland. Forskolin (10 microM) and isoproterenol (10 microM) elevated cAMP in the parotid gland by 2-fold and 7-fold, respectively, and these agents also inhibited oxotremorine-M (3 microM) mediated phosphoinositide hydrolysis by 14% and 26%, respectively. Forskolin (1, 4.3, 18, and 75 microM) increased cAMP accumulation and inhibited PIP2 hydrolysis in a concentration-dependent manner. Forskolin (75 micrometers) shifted the concentration-response curve for the full agonist oxotremorine-M rightward by 4.2-fold. Pre-treatment with the phosphodiesterase inhibitor isobutylmethylxanthine (1 mM) reduced the maximum effect of oxotremorine-M by 31%. The inhibitory effect of isoproterenol and forskolin on muscarinic receptor-mediated phosphoinositide hydrolysis was unaffected by the removal of extracellular Ca2+. Moreover, isoproterenol and forskolin dampened sodium fluoride and oxotremorine-M mediated phosphoinositide hydrolysis to the same extent suggesting that the inhibitory effect of cAMP is downstream from the muscarinic receptor.

Heparin, dextran and trypan blue allosterically modulate M2 muscarinic receptor binding properties and interfere with receptor-mediated inhibition of adenylate cyclase

The influences of heparin, dextran and trypan blue on muscarinic receptor binding properties and inhibition of adenylate cyclase were investigated in homogenates of the rat heart. These compounds caused a concentration-dependent enhancement in the specific binding of the muscarinic antagonist [3H]N-methylscopolamine ([3H]NMS) when measured at a radioligand concentration of approximately 0.5 nM in magnesium-containing, low ionic strength buffer. The maximal enhancements of [3H]NMS binding were 2.89-, 1.68- and 1.43-fold increases for heparin, dextran and trypan blue, respectively; the EC50 values for this effect were 0.12, 0.033 and 4.6 microM, respectively. The effects of heparin, dextran and trypan blue on [3H]NMS binding were attributed mainly to an increase in the overall affinity of muscarinic receptors for [3H]NMS, and were greatly attenuated by 100 mM NaCl. These effects were qualitatively similar to those produced by GTP. Heparin, dextran and trypan blue also affected the binding of the muscarinic agonist oxotremorine-M in a manner similar to that of GTP; that is, in the presence of these compounds, agonist affinity was decreased. Our experiments also showed that heparin and dextran attenuate the inhibition of adenylate cyclase activity caused by oxotremorine-M in myocardial homogenates without influencing basal adenylate cyclase activity. We conclude that heparin and dextran interfere with the muscarinic receptor-G protein coupling in the rat heart.