K and a Opioid Receptor Stimulation Affects Cardiac Myocyte Function and Ca 2 ' Release From an Intracellular Pool in Myocytes and Neurons

We investigated the effects of,, 8, and #c opioid receptor stimulation on the contractile properties and cytosolic Ca2' (Cal) of adult rat left ventricular myocytes. Cells were fieldstimulated at 1 Hz in 1.5 mM bathing Ca2+ at 23°C. The,u-agonist [D-Ala2,N-Me-Phe4,Gly5oll-enkephalin (10-` M) had no effect on the twitch. The a-agonists methionine enkephalin and leucine enkephalin (10-10 to 10-6 M) and the c-agonist (trans-(dl)-3,4-dichloro-N-methyl-N[2(1-pyrrolidinyl)cyclo-hexyll-benzeneacetamide)methanesulfonate hydrate (U-50,488H; 10-7 to 2x 10-5 M) had a concentration-dependent negative inotropic action. The sustained decrease in twitch amplitude due to U-50,488H was preceded by a transient increase in contraction. The effects of tand K-receptor stimulation were antagonized by naloxone and (-)-N-(3-furylmethyl)-ce-normetazocine methanesulfonate, respectively. In myocytes loaded with the Ca21 probe indo-1, the effects of leucine enkephalin (10-` M) and U-50,488H (10-` M) on the twitch were associated with similar directional changes in the Caj transient. Myofilament responsiveness to Ca2' was assessed by the relation between twitch amplitude and systolic indo-1 transient. Leucine enkephalin (10-8 M) had no effect, whereas U-50,488H (10-` M) increased myofilament responsiveness to Ca2`. We subsequently tested the hypothesis thata andic opioid receptor stimulation may cause sarcoplasmic reticulum Ca2' depletion. The sarcoplasmic reticulum Ca21 content in myocytes and in a caffeine-sensitive intracellular Ca2' store in neurons was probed in the absence of electrical stimulation via the rapid addition of a high concentration of caffeine from a patch pipette above the cell. U50,488H and leucine enkephalin slowly increased Ca; or caused Caj oscillations and eventually abolished the caffeine-triggered Caj transient. These effects occurred in both myocytes and neuroblastoma-2a cells. In cardiac myocyte suspensions U-50,488H and leucine enkephalin both caused a rapid and sustained increase in inositol 1,4,5-trisphosphate. Thus, and Pc but not , opioids have a negative inotropic action due to a decreased Ca, transient. The decreased twitch amplitude due to i-receptor stimulation is preceded by a transient increase in contractility, and it occurs despite an enhanced myofilament responsiveness to Ca2`. The effects of a and #c opioids appear coupled to phosphatidylinositol turnover and, at least in part, may be due to sarcoplasmic reticulum Ca2' depletion. Ca2' release and depletion of an intracellular store site occur in both myocytes and neurons and may represent a general mechanism for the effects of opioids. (Circulation Research 1992;70:66-81)