Effects of pharmacological preconditioning with U50488H on calcium homeostasis in rat ventricular myocytes subjected to metabolic inhibition and anoxia

1. The effects of pharmacological preconditioning with U50488H (U ₅₀), a selective κ-opioid receptor agonist, on Ca ²⁺ homeostasis in rat ventricular myocytes subjected for 9 min to metabolic inhibition (MI) and anoxia (A), consequences of ischaemia, were studied and compared with those of preconditioning with brief periods of MI/A. 2. Precondition with 30 μM of U ₅₀ for three cycles of 1 min each cycle separated by 3 min of recovery (UP) significantly increased the percentage of non-blue cells following MI/A. The effect of UP is the same as that of preconditioning with an inhibitor of glycolysis and an oxygen scavenger for three 1-min cycles separated by three-minute recovery (MI/AP). The results indicate that like MI/AP, UP also confers cardioprotection. 3. MI/A increased intracellular Ca ²⁺ ([Ca ²⁺] _i) and reduced the amplitude of caffeine-induced [Ca ²⁺] _i transients, an indication of Ca ²⁺ content in the sarcoplasmic reticulum (SR). MI/A also reduced the electrically-induced [Ca ²⁺] _i transient, that indicates Ca ²⁺-release during excitation-contraction coupling, and Ca ²⁺ sparks in unstimulated myocytes, that indicates spontaneous Ca ²⁺-release from SR. It also prolonged the decline of the electrically-induced [Ca ²⁺] _i transient and slowed down the recovery of the electrically-induced [Ca ²⁺] _i transient after administration of caffeine. In addition, MI/A prolonged the decline of caffeine induced [Ca ²⁺] _i transient, an indication of Na ⁺-Ca ²⁺ exchange activity, and UP prevented it. So UP, that confers cardioprotection, prevented the changes induced by MI/A. With the exception of Ca ²⁺-spark, which was not studied, the effects of MI/AP are the same as those of UP. 4. It is concluded that pharmacological preconditioning with U ₅₀, that confers immediate cardioprotection, prevents changes of Ca ²⁺ homeostasis altered by MI/A in the rat heart. This may be responsible, at least partly, for the cardioprotective action. 5. The study also provided evidence that MI/A causes mobilization of Ca ²⁺ from SR to cytoplasm causing Ca ²⁺-overload which may be due to reduced Ca ²⁺-uptake by SR. MI/A also reduces spontaneous and electrically induced Ca ²⁺ release from SR.