Sigma receptor ligands modulate contractility, Ca⁺⁺ influx and beating rate in cultured cardiac myocytes

Specific binding of [³H]-1,3-di-o-tolylguanidine (DTG) and (+)-[³H]-(3- hydroxyphenyl)-N-(1-propyl)-piperidine [(+)-3-PPP] to membranes of cultured cardiac myocytes from neonatal rats revealed the presence of sigma receptors on these cells. Exposure of cultured cardiomyocytes to nanomolar concentrations of (+)-3-PPP, (+)-pentazocine and haloperidol induced specific patterns of changes in contractility of electrically paced cultures. The amplitude of systolic cell-motion (ASM) decreased by 10 to 25% 1 to 2 min after drug addition, then transiently increased (3-10 min) and finally decreased to about 75% of control level. Fluorescence measurements on indo-1 loaded cardiomyocytes revealed drug-induced changes in the size of the concentration of free cytosolic calcium ([Ca⁺⁺](i))-transients, similar to the changes observed in ASM. These changes appear to be mediated by corresponding changes in the rates of ⁴⁵Ca⁺⁺ influx which increased 2 to 7 min after the addition of (+)-3-PPP and decrease to 50% of the control level thereafter. Preincubation with thapsigargin, which depletes the sarcoplasmic reticulum-Ca⁺⁺ stores, did not affect the pattern of changes in ASM, induced by the subsequent addition of (+)-3-PPP. This indicates that the changes in [Ca⁺⁺](i) are not mediated by sarcoplasmic reticulum-Ca⁺⁺ transport systems. Exposure to sigma ligands did not affect the apparent sensitivity of the myofilaments to Ca⁺⁺, as indicated by the relationships between changes in ASM and in [Ca⁺⁺](i)-transients. Cultures which were not paced, contracted spontaneously at a constant rhythm. Sigma receptor, ligands caused changes in beating frequencies which were followed by irregular contractions. The present work suggests that sigma receptors may have an important role in regulating contractility, beating rates and Ca⁺⁺ fluxes in cardiac myocytes.