κ and δ opioid receptor stimulation affects cardiac myocyte function and Ca2+ release from an intracellular pool in myocytes and neurons

Abstract

We investigated the effects of μ, δ, and κ opioid receptor stimulation on the contractile properties and cytosolic Ca2+ (Cai) of adult rat left ventricular myocytes. Cells were field-stimulated at 1 Hz in 1.5 mM bathing Ca2+ at 23°C. The μ-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10-5 M) had no effect on the twitch. The δ-agonists methionine enkephalin and leucine enkephalin (10-10 to 10-6 M) and the κ-agonist (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclo-hexyl]- benzeneacetamide)methanesulfonate hydrate (U-50,488H; 10-7 to 2×10-5 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 δ- and κ-receptor stimulation were antagonized by naloxone and (-)-N-(3-furyl-methyl)-α-normetazocine methanesulfonate, respectively. In myocytes loaded with the Ca2+ probe indo-1, the effects of leucine enkephalin (10-8 M) and U-50,488H (10-5 M) on the twitch were associated with similar directional changes in the Cai 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 (10-5 M) increased myofilament responsiveness to Ca2+. We subsequently tested the hypothesis that δ and κ opioid receptor stimulation may cause sarcoplasmic reticulum Ca2+ depletion. The sarcoplasmic reticulum Ca2+ 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. U-50,488H and leucine enkephalin slowly increased Cai or caused Cai oscillations and eventually abolished the caffeine-triggered Cai 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, δ and κ but not μ opioids have a negative inotropic action due to a decreased Cai transient. The decreased twitch amplitude due to κ-receptor stimulation is preceded by a transient increase in contractility, and it occurs despite an enhanced myofilament responsiveness to Ca2+. The effects of δ and κ 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.