Opioid drugs are known to affect several hemodynamic parameters in the cardiovascular system. The experimental results of our previous studies proved direct effects of opiates on the cardiac muscle. These effects were stereo-selective and blocked by specific agonists, indicating the involvement of specific opioid receptors. The present study sought to identify the cellular effects of opiates and to reveal the signal transduction pathway, which mediates these effects. We were especially interested in the stimulatory effects of opioid agonists on intracellular calcium metabolism due to the important role of calcium ions in the regulation of cardiac activity.
Free Ca++ was measured by single cell imaging using the fluorescent indicator fura-2. Intracellular pools of Ca++ were not involved since the stimulatory opioid activity resisted unloading by ryanodine, tapsygargin and caffeine. On the other hand, extracellular Ca++ was crucial since its removal completed blocked the effect. Calcium entered the cardiomyocytes through voltage-dependent Ca++ channels which could be blocked by verapamil. Introducing the selective blockers W-7 and chelerythrine proved the involvement of intracellular calmodulin and protein kinase C, respectively.
In order to explore the mechanism by which opioid receptors activate the intracellular machinery, we studied the coupling between the receptors and GTP-binding proteins in the cardiac cell membrane. For this purpose, the binding of [35S]GTP”S to isolated membranes was measured. Opioid agonists stimulated binding in a stereoselective and selective fashion. This stimulation was abolished by pertussis toxin, indicating the involvement of Gi/Go proteins. Thus the first step in opioid signal transduction pathway in the cardiac muscle was found to be similar to that in neuronal tissue.
We are currently studying the long-term effects of opioid drugs on cardiac muscle. Our preliminary experiments demonstrate down-regulation of opioid receptors and desensitization of opioid activity following prolonged exposure to opioid agonists. The implication of these findings to chronic effects of opiates, either in medical use or as drugs of abuse, on cardiovascular functions awaits further in vivo experiments.
