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-opioid receptor stimulation is
associated with a slowing of cross-bridge cycling
Department of Physiology, University of Tennessee, Memphis, Tennessee 38163
Opioid and
-adrenergic receptor activation protect the heart from ischemic
damage. One possible intracellular mechanism to explain this is that an
improvement in ATP availability contributes to cardioprotection. We
tested this hypothesis by correlating postischemic left ventricular
developed pressure (LVDP) and myofibrillar Ca2+-dependent
actomyosin Mg2+-ATPase from isolated rat hearts treated
with the
-opioid receptor agonist U-50488H (1 µM) or the
-adrenergic receptor agonist phenylephrine (10 µM) + propranolol (3 µM). Preischemic treatment with U-50488H or
phenylephrine + propranolol improved postischemic LVDP recovery by
25-30% over control hearts. Ca2+-dependent actomyosin
Mg2+-ATPase was found to be 20% lower in both U-50488H-
and phenylephrine + propranolol-treated hearts compared with
control hearts. The
-opioid receptor antagonist
nor-binaltorphimine (1 µM) abolished the effects of U-50488H
on postischemic LVDP and actomyosin Mg2+-ATPase activity.
Reduced actomyosin ATP utilization was also suggested in single
ventricular myocytes treated with either U-50488H or the protein kinase
C activator, phorbol 12-myristate 13-acetate (PMA), because U-50488H
and PMA lowered maximum velocity of unloaded shortening by 15-25%
in myocytes. U-50488H and phenylephrine + propranolol treatment
both resulted in increased phosphorylation of troponin I and C protein.
These findings are consistent with the hypothesis that
-opioid and
-adrenergic receptors decrease actin-myosin cycling rate, leading to
a conservation of ATP and cardioprotection during ischemia.
preconditioning; U-50488H; myofibrillar magnesium adenosine 5'-triphosphatase; metabolic slowing; myocytes; cardiac; velocity of shortening; left ventricular developed pressure; troponin I; C protein
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