Detection of skeletal muscle calcium channel subunits in cultured neonatal rat cardiac myocytes.

A monoclonal antibody (mAb) 8B7, that recognizes specifically the alpha 1-subunit of dihydropyridine-sensitive calcium channels in skeletal muscle, exerted a dose-dependent positive chronotropic effect on cultured spontaneously beating neonatal rat cardiac myocytes. The antibody-induced increase in beating frequency was similar to that elicited by 1 microM of (-)-isoprenaline. The mAb 8B7 recognized a polypeptide that is analogous to the 170-kDa calcium channel alpha 1-subunit of rabbit skeletal muscle in wheat germ lectin purified proteins solubilized from cultured rat heart myocytes by Western blot analysis. Furthermore, the mAb 7C3, previously shown to recognize specifically the skeletal muscle calcium channel beta-subunit, reacted with a 55-kDa polypeptide in these cultured myocytes. No immunoreactivity, however, was obtained when proteins solubilized from both neonatal or adult rat heart tissue were employed in Western blotting or immunoprecipitation experiments. RNA-hybridization analysis revealed that primary cultures derived from neonatal rat hearts expressed two types of mRNA encoding the skeletal muscle as well as the cardiac calcium channel alpha 1-subunit, while neonatal rat heart tissue expressed only the message for cardiac calcium channel. These results provide evidence for a functional expression of skeletal muscle-type calcium channel subunits during the cultivation of neonatal rat heart cells.