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Am J Physiol Heart Circ Physiol 281: H669-H678, 2001;
0363-6135/01 $5.00
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Vol. 281, Issue 2, H669-H678, August 2001

Effects of kappa -opioid receptor activation on myocardium

W. G. Pyle, J. W. Lester, and P. A. Hofmann

Department of Physiology, University of Tennessee, Memphis, Tennessee 38163

kappa -Opioid receptor stimulation of the heart transiently increases twitch amplitude and decreases Ca2+-dependent actomyosin Mg2+-ATPase activity through an undetermined mechanism. One purpose of the present study was to determine if the increase in twitch amplitude is due to changes in myofilament Ca2+ sensitivity. We also wanted to determine if kappa -opioid receptor activation alters maximum actin-myosin ATPase activity and Ca2+ sensitivity of tension in a way consistent with protein kinase A or protein kinase C (PKC) action. Rat hearts were treated with U50,488H (a kappa -opioid receptor agonist), phenylephrine plus propranolol (alpha -adrenergic receptor stimulation), isoproterenol (a beta -adrenergic receptor agonist), or phorbol 12-myristate 13-acetate (PMA, receptor independent activator of PKC) or were untreated (control), and myofibrils were isolated. U50,488H, phenylephrine plus propranolol, and PMA all decreased maximum Ca2+-dependent actomyosin Mg2+-ATPase activity, whereas isoproterenol treatment increased maximum Ca2+-dependent actomyosin Mg2+- ATPase activity. Untreated myofibrils exposed to exogenous PKC-epsilon , but not PKC-delta , decreased maximum actomyosin Mg2+-ATPase activity. Langendorff-perfused hearts treated with U50,488H, phenylephrine plus propranolol, or isoproterenol had significantly higher ventricular ATP levels compared with control hearts. PKC inhibitors abolished the effects of U50,488H on Ca2+-dependent actomyosin Mg2+-ATPase activity and myocardial ATP levels. U50,488H and PMA treatment of isolated ventricular myocytes increased Ca2+ sensitivity of isometric tension compared with control myocytes at pH 7.0. The U50,488H-dependent increase in Ca2+ sensitivity of tension was retained at pH 6.6. Together, these findings are consistent with the hypotheses that 1) the positive inotropy associated with kappa -opioid receptor activation may be due in part to a PKC-mediated increase in myofilament Ca2+-sensitivity of tension and 2) the kappa -opioid receptor-PKC pathway is a modulator of myocardial energy status through reduction of actomyosin ATP consumption.

calcium sensitivity of tension; pH; ATP; protein kinase C; actomyosin Mg2+-ATPase


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