Apparent up-regulation of stimulatory G-protein alpha subunits in the pregnant human myometrium is mimicked by elevated smoothelin expression

Sensitization of adenylyl cyclase (AC) by increased expression of large iso forms of the stimulatory G-protein G alpha(s) has been suggested as a mecha nism that governs uterine quiescence during pregnancy. We quantified severa l components of the AC pathway in pregnant (P, n=21) and nonpregnant human myometria (NP, n=10). AC activity was similar to sevenfold higher in P than in NP under basal and stimulated conditions (MnCl2/GTP/GTP + isoproterenol ). in addition, relative stimulation (% of basal) by 5'-guanosine-beta gamm a-iminotriphosphate and forskolin was twofold higher in P. beta-Adrenocepto r density was low and unaltered in P. G alpha(s) mRNA splice variants did n ot differ in P. Using antisera against different epitopes of G alpha(s) (ca rboxyl-/more aminoterminal), we found unchanged expression of G alpha(s) sh ort and long (45, 47 kDa) in P. Two additional proteins in P (51, 59 kDa) w ere detectable only by the carboxyl-terminal antiserum and lacked GTP bindi ng properties. The 59 kDa protein could be identified as a recently discove red cytoskeletal protein, smoothelin, which was 10-fold increased in P. The se data indicate that the apparent up-regulation of large G alpha(s) specie s in P is mimicked by elevated smoothelin. Therefore, the increase in AC ca nnot be attributed to changes in G alpha(s)- or beta-adrenoreceptors. Epito pe sharing between G alpha(s), and smoothelin should be considered in exper iments on smooth muscle tissues.-Gsell, S., Eschenhagen, T., Kaspareit, G., Nose, M., Scholz, H., Behrens, O., Wieland, T. Apparent up-regulation of s timulatory G-protein alpha subunits in the pregnant human myometrium is mim icked by elevated smoothelin expression.