INOTROPIC ACTION OF SIGMA-RECEPTOR LIGANDS IN ISOLATED CARDIAC MYOCYTES FROM ADULT-RATS

High affinity binding sites for a receptor ligands were found in membranes of cardiac myocytes from adult rats. The sigma receptor ligand (+)-3-hydroxyphenyl-N-(1-propyl)piperidine ((+)-3-PPP) binds with a K-d of 17.9 +/- 4.0 nM and a B-max of 275 +/- 32.1 fmol/mg protein. Competition experiments of (+)-pentazocine with [H-3]1,3-di-O-tolylguanidine ([H-3]DTG) binding yielded a K-i of 6.1 +/- 1.3 nM. The majority of the sites (> 80%) were of the sigma(1) subtype. Exposure of isolated cardiomyocytes from adult rats to (+)-3-PPP (10 nM-1.0 mu M) caused a marked concentration-dependent increase in the amplitude of systolic cell contraction, reaching 149% of control level, with an apparent ED(50) value of 4.5 nM. The increase in the contraction amplitude was markedly inhibited by pretreatment with verapamil or thapsigargin. An increase in the amplitude of [Ca2+](i) transients, similar to that in the amplitude of cell contraction, was observed in indo-1-loaded cardiomyocytes exposed to 0.1 mu M (+)-3-PPP. Exposure to 10 nM of haloperidol or (+)-pentazocine induced an increase in the amplitude of contraction, reaching 188% and 138% (respectively) of control level. A lower concentration of haloperidol or (+)-pentazocine (1 nM) did not induce an increase in the contraction amplitude but rather reduced the amplitude to 70-80% of control.