Forskolin: Unique diterpene activator of adenylate cyclase in pregnant and nonpregnant guinea pig myometrial membranes

Abstract

In guinea pig myometrium, β-adrenergic receptors are functionally coupled to adenylate cyclase. β-Adrenergic receptor agonists in the presence of guanosine triphosphate stimulate adenylate cyclase activity, thus increasing 3′5′-cyclic adenosine monophosphate synthesis and promoting myometrial relaxation. In pregnant animals close to term (65 days), β-adrenergic receptor density as well as basal and (−)isoproterenol-dependent (in the presence of guanosine triphosphate) adenylate cyclase activity is significantly higher than that in nonpregnant animals or those in early pregnancy. Since this system appears to be made up of at least three components (β-adrenergic receptor, guanosine triphosphate-binding protein, and a catalytic component), these observations on total adenylate cyclase activity may reflect alterations in one or more of these components. To answer the question whether the catalytic unit of this system can be directly assayed and whether its activity is influenced by pregnancy, we have performed in vitro experiments to measure the enzymatic activity of the catalytic component of the β-adrenergic receptor-adenylate cyclase complex in guinea pig myometrial membranes. We have used two compounds that stimulate the catalytic component: forskolin and manganese chloride. Forskolin, regardless of the presence or absence of guanosine triphosphate, is the most potent stimulator of adenylate cyclase activity in myometrial membranes from nonpregnant and pregnant animals; manganese chloride is a less potent activator. The degree of adenylate cyclase stimulation by forskolin tends to be higher in uteri from pregnant (≥0.5 gestation) than from nonpregnant or postpartum animals. It was concluded: (1) that adenylate cyclase stimulation by forskolin does not depend on the presence of β-adrenergic receptor agonists or guanosine triphosphate and (2) that with advancing gestation there might be a qualitative or quantitative change with regard to the interaction between forskolin and the presumed catalytic component of the β-adrenergic receptor-adenylate cyclase complex.