Abstract
α1-Adrenergic receptor stimulation can inhibit the Cl− current activated by β-adrenergic receptor agonists in guinea-pig ventricular myocytes. We investigated the role of G proteins in mediating this type of α-adrenergic response. The combined α- and β-adrenergic agonist norepinephrine (NE) activated the Cl− current with an EC50 value of 53 nm. Preincubation of myocytes with PTX decreased the EC50 value for NE activation of the Cl−current to 5.9 nm, and addition of the α1-adrenergic receptor antagonist prazosin did not cause any further change in sensitivity to NE. These results suggest that the α1-adrenergic inhibition of β-adrenergic responses is mediated through a PTX-sensitive G protein. However, PTX pretreatment also increased the sensitivity of the Cl− current to the selective β-adrenergic agonist isoproterenol (Iso), which indicates that the PTX treatment increases the sensitivity to β-adrenergic stimulation alone and that this could account for the PTX-induced change in sensitivity to NE. Consistent with this idea, the selective α1-adrenergic receptor agonist methoxamine was still able to inhibit the Cl− current activated by Iso in PTX-treated myocytes. However, the sensitivity to methoxamine was significantly decreased. In control cells, the Cl− current activated by 30 nm Iso was inhibited by methoxamine with an EC50 value of 8.3 μm, but in PTX-treated cells, the EC50 value was 284 μm. The EC50 for methoxamine inhibition was similarly increased when the Cl− current was activated by 300 nmIso. These data suggest that the effects of PTX on α1-adrenergic responses can actually be explained by changes in the sensitivity to β-adrenergic stimulation. To verify the role for a G protein in mediating the inhibitory α1-adrenergic response, we examined the effect of methoxamine on the Cl− current activated in cells dialyzed with the nonhydrolyzable GTP analogue guanosine-5′-O-(3-thio)triphosphate. Pre-exposure to methoxamine resulted in an attenuated response upon subsequent exposure to Iso alone. We conclude that α1-adrenergic inhibition of β-adrenergic responses is mediated by a G protein-dependent mechanism that appears to be PTX-insensitive.
Footnotes
- Received November 21, 1996.
- Accepted January 16, 1997.
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Send reprint requests to: Dr. Robert D. Harvey, Department of Physiology and Biophysics, Case Western Reserve University, 2109 Adelbert Road, Cleveland, OH, 44106-4970. E-mail: rdh3{at}po.cwru.edu
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This work was supported by a grant from the National Institutes of Health (HL45141), an Established Investigatorship from the American Heart Association (R.H.), and a Postdoctoral Fellowship from the Northeast Ohio Affiliate of the American Heart Association (L.H.).
- The American Society for Pharmacology and Experimental Therapeutics
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