Stimulation of opioid receptors on cardiac ventricular myocytes reduced L type Ca2+ channel current

Recent studies have indicated that opioid peptide receptors are present on cardiac ventricular cells and that Leucine enkephalin (LE), a naturally occurring δ opioid peptide receptor agonist, leads to marked reductions in twitch amplitude and in the cytosolic Ca²⁺ transient (Ca(i)) of single adult rat ventricular myocytes. The specific mechanisms by which Ca(i) is reduced by LE have not been fully elucidated. Specifically, it is unknown whether LE affects the Ca²⁺ current (I(Ca)) of L type Ca²⁺ channels. In the present study we determined the effect of LE on I(Ca) of individual cardiac ventricular cells freshly isolated from adult rats. LE (10^-8 M) decreased the amplitude of I(Ca) by 40% (during regular whole cell voltage clamp depolarizations to 0 mV at 0.5 Hz at 23°C from a holding potential of -40 mV). The relative magnitude of this effect increased with the magnitude of the test potential from -20 to +50 mV. I(Ca) inactivation was also prolonged by LE. These effects of LE on I(Ca) were abolished by Naloxone (NAL), an opioid receptor antagonist. Thus, the effects of the opioid peptide, LE to decrease the Ca transient and contraction amplitudes in individual cardiac ventricular cells, are, in part, mediated by an LE induced reduction in I(Ca).