UNEXPECTED CONTRACTILE RESPONSE FROM THE G-PROTEIN ACTIVATOR MASTOPARAN IN CARDIAC MYOCYTES

Mastoparan preferentially activates the G protein Gi in transmembrane signalling pathways. In heart tissue Gi couples receptors to negative inotropy so we have examined the effect of mastoparan on contractility in cardiac myocytes electrically stimulated at 0.5 Hz. Mastoparan (10 (-5) mol/l) caused a time-dependent increase in cell shortening to a m aximum of 83.7 +/- 18.0% (n = 7) of the resting cell length at between 4 and 7 min. This could be the result of either stimulation of other G proteins or the release of betagamma subunits from Gi which activate phospholipase C.