κ-Opioid peptide receptor stimulation increases cytosolic pH and myofilament responsiveness to Ca²⁺ in cardiac myocytes

Although κ- and δ-opioid receptors on mammalian cardiac myocytes have been discovered recently, the intracellular effects that result from stimulation of these receptors remain unknown. We examine the effects of a rapid and brief exposure to a κ-opioid receptor agonist on intracellular Ca²⁺, pH, and cell length in individual isolated rat ventricular cells. The specific κ-agonist trans-dl-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]- benzene-acetamide (U-50488H) (methane sulfonate salt) caused a transient increase in cytosolic pH (pH(i)) measured from the change in SNARF-1 fluorescence and an increase in cytosolic [Ca²⁺] (Ca(i)), indexed by a change in indo-1 fluorescence. The initial Ca(i) increase often was followed by Ca(i) oscillations. Both pH(i) and Ca(i) effects were blocked by the specific antagonist κ-opioid receptor l-(N-furylmethyl)-α-normetazocine methane-sulfonate (Mr 1452). The amplitude of contraction that accompanied the Ca(i) increase elicited by U-50488H was greater than that associated with a similar increase in Ca(i) elicited by electrical stimulation or by the rapid exposure of cells to caffeine. Thus an acute and brief κ-opioid receptor stimulation of cardiac cells leads to an increase in Ca(i) and pH(i). The pH(i) increase was abolished by 1) blockade of the Na⁺-H⁺ exchanger by ethyl isopropyl amiloride and 2) inhibition of protein kinase C (PKC) activity via pretreatment with staurosporine or prolonged incubation with 4β-phorbol 12-myristate 13-acetate. These maneuvers did not abolish the U-50488H-induced increase in Ca(i). Thus the κ-opioid-induced increase in pH(i) and associated enhanced myofilament response to Ca²⁺ in individual heart cells likely is mediated, at least in part, via PKC activation of Na⁺-H⁺ exchange.