title: Alpha 1-adrenergic receptor stimulation decreases maximum shortening velocity of skinned single ventricular myocytes from rats.
abstract: alpha 1-Adrenergic agonists have negative inotropic effects on mammalian myocardium under some conditions, and biochemical experiments measuring the Ca(2+)-activated actomyosin ATPase activity of myofibrillar preparations suggest that this may result from a decrease in cross-bridge cycling rate caused by phosphorylation of myofilament proteins. Experiments with intact ventricular preparations, however, have failed to demonstrate a mechanical manifestation of a decrease in cycling rate. The present study examined the effect of alpha 1-adrenergic receptor stimulation on maximum shortening velocity in skinned single ventricular myocytes from rats. Enzymatically isolated myocytes were incubated with the beta-receptor antagonist propranolol in the presence or absence of the alpha 1-adrenergic receptor agonist phenylephrine and were then rapidly skinned to preserve the phosphorylation state of myofilament proteins. The velocity of unloaded shortening (Vo) was determined by use of the slack-test method and compared between skinned control and phenylephrine-treated cells. The relationship between isometric tension and [Ca2+] was also assessed for each myocyte. Vo was significantly lower in the alpha 1-adrenergic receptor agonist-treated cells than in the control cells, but there was no effect on Ca2+ sensitivity of isometric tension. In addition, the myosin heavy chain isoform composition accounted for a significant amount of the variation in Vo within the treatment groups. On the basis of these and previous results we propose that alpha 1-adrenergic receptor stimulation inhibits cross-bridge cycling rate at the level of myofilament proteins by a mechanism that may involve phosphorylation of troponin I by protein kinase C.
authors.affiliation: Department of Physiology, School of Medicine, University of Wisconsin, Madison, USA.
authors.forename: K T
authors.initials: KT
authors.lastname: Strang
authors.forename: R L
authors.initials: RL
authors.lastname: Moss
mesh_terms: D008841:Actin Cytoskeleton / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism; D002469:Cell Separation; D004591:Electrophoresis, Polyacrylamide Gel; D005260:Female; D066298:In Vitro Techniques; D007537:Isometric Contraction; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009206:Myocardium / Q000166:cytology* / Q000378:metabolism; D009218:Myosins / Q000378:metabolism; D010656:Phenylephrine / Q000494:pharmacology; D010766:Phosphorylation; D011433:Propranolol / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D011942:Receptors, Adrenergic, alpha / Q000187:drug effects / Q000502:physiology*; D013268:Stimulation, Chemical
chemical_list: D011942:Receptors, Adrenergic, alpha; D010656:Phenylephrine; D011433:Propranolol; D009218:Myosins; D002118:Calcium