The generation of the second messengers inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG) by phosphoinositide-specific phospholipases C (PLCs) is a key mechanism by which many cellular functions such as intracellular calcium handling or growth and differentiation are modulated. In the myocardium, PLC plays a role in the mediation of positive inotropic effects and is possibly involved in the pathogenesis of myocardial hypertrophy. Among the variety of PLC isozymes known, the PLC beta family is regulated by heterotrimeric G proteins. The aim of the present study was to identify and to characterize the PLC beta isoform present in human myocardium. PLC activity in human myocardial membranes was dependent on the presence of Ca2+. Interestingly, PLC was markedly stimulated by GTP gamma S, used as an activator of G proteins. This stimulation was completely abolished by GDP. However, purified alpha-subunits from retinal transducin (alpha 1), used as scavengers of free beta gamma-subunits, did not abolish this effect indicating GTP gamma S stimulation being mediated by G protein alpha-subunits. PLC activity was also stimulated by G protein beta gamma-subunits purified from bovine retina (beta gamma t). This stimulation was completely blocked by addition of purified alpha t. Reverse transcriptions and polymerase chain reactions (RT-PCR) provided evidence for PLC beta 1 mRNA being expressed in human myocardium, whereas PCR products corresponding to PLC beta 2 and PLC beta 3 mRNAs were not detected. It is concluded that PLC beta 1 mRNA is expressed in human myocardium. The functional properties of human myocardial PLC activity correspond well to the properties established for PLC beta 1, i.e. sensitivity to G protein alpha-as well as beta gamma-subunits. The presence of other as yet unidentified PLC isozymes is nevertheless possible. The identification of the PLC beta isozyme present in human myocardium and the understanding of its regulation by G protein subunits sets the stage for the investigation of possible involvement of this system in the pathophysiology of myocardial hypertrophy.