While it has been well established that, in vertebrate skeletal muscle, Caions activating the contractile system (activator Ca) are released from the sarcoplasmic reticulum (SR), the source of activator Ca in various types of smooth muscle still remain a matter of debate and speculation. In smooth muscle, the SR is much less prominent than in skeletal muscles. Some types of smooth muscle, however, are markedly resistant to removal of external Ca ions and can be activated to contract by various agents in Ca-free media, indicating the presence of intracellularly stored Ca ions (Atsumi and Sugi, J.Physiol., 257,549-560,1976, Sugi and Daimon, Nature, 269,436-438,1997, Debbas et al., Anat.Rec, 182,447-472,1975).
To search for the candidates for the Ca^<2+>-accumulator, we surveyed Ca^<2+>-binding proteins (CaBPs) in rabbit vascular smooth muscles (A.basilais, A.carotis communis, Pars ascendens aortae, Pars abdominalis aortae and A.femoralis) by means of SDS-poylacrylamide gel electrophoresis combined with ^<45>Ca autoradiography. ^<45>Ca radioactive bands are observed in proteins bands with molecular weights of about 55,000,39,000,25,000and 17,000 in all the vascular samples studied. And the same pattern of ^<45>Ca autoradiograph was observed in bovine and porcine vascular smooth muscles.
Plasma membrane-enriched (PM) and sarcoplasmic reticulum-enriched (SR) fractions were prepared from Bovine Aorta smooth muscle by differential centrifugations followed by discontinuous sucrose density gradients. ^<45>Ca radioactive bands are observed in proteins bands with molecular weights of about 110,000,55,000,25,000 and 17,000 in microsomal fraction, 110,000 and 42,000 in PM fraction and 110,000,55,000 and 42,000 in SR fraction
These results suggest that the plasma membrane may be involved role in regulating intracellular Ca^<2+> in vascular smooth muscle.