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Halothane inhibition of canine myocardial adenylate cyclase--modulation by endogenous factors
Insights
Halothane
Area of Science:
- Cardiovascular Physiology
- Anesthesiology
- Molecular Pharmacology
Background:
- Halothane is known to depress myocardial contractility.
- Previous studies suggested halothane's depression of adenylate cyclase contributes to this effect.
- Adenylate cyclase activity is crucial for cardiac function and regulation.
Purpose of the Study:
- To investigate the role of canine myocardial adenylate cyclase in halothane-induced myocardial depression.
- To determine if halothane directly affects adenylate cyclase activity in isolated sarcolemmal membranes.
- To identify potential modulators involved in halothane's action on myocardial adenylate cyclase.
Main Methods:
- Isolated canine myocardial sarcolemmal membranes were used to assay adenylate cyclase activity.
- Catecholamine- and guanosine triphosphate (GTP)-stimulated adenylate cyclase activity was measured in the presence of halothane.
- Reconstitution experiments were performed with an adenylate cyclase-free supernatant to identify modulators.
Main Results:
- Halothane did not inhibit catecholamine-stimulated adenylate cyclase in isolated sarcolemmal membranes.
- Reconstitution with a supernatant restored halothane's inhibitory effect on GTP-stimulated adenylate cyclase.
- The magnitude of halothane's depression was dose-dependent on the supernatant concentration, indicating the presence of modulators.
Conclusions:
- Endogenous modulators present in the myocardial supernatant are essential for halothane to depress adenylate cyclase activity.
- These modulators can be reversibly separated from the adenylate cyclase complex.
- The findings suggest a complex mechanism involving modulators, not just direct enzyme inhibition, underlies halothane's cardiac effects.
Abstract:
We have hypothesized that the halothane-induced depression of myocardial contractility can be explained, at least in part, by halothane's depression of adenylate cyclase, previously demonstrated in whole homogenates of myocardial tissue. Canine myocardial sarcolemmal membranes, which contain the adenylate cyclase of myocardial cells, were separated from other cellular constituents. Halothane did not depress catecholamine-stimulated adenylate cyclase activity in this preparation. Reconstitution of the sarcolemmal membrane preparation with a 100,000 X g adenylate cyclase-free supernatant restored the depressant effect of halothane on adenylate cyclase stimulated by guanosine triphosphate (GTP) 100 microM alone (-55%, P less than 0.01) or in combination with l-isoproterenol 1 microM (-38%, P less than 0.05) or 2.5 microM (-40%, P less than 0.01). Dilution of the supernatant to half-strength decreased the magnitude of the halothane-induced depression of adenylate cyclase activity to 19% (P less than 0.01); at one-quarter dilution, the effect was no longer significant. This study demonstrates the presence of endogenous modulators of the action of halothane on canine myocardial adenylate cyclase that can be reversibly separated from the adenylate cyclase complex.