Differential alterations in responsiveness in particulate and soluble guanylate cyclases in pregnant guinea pig myometrium

OBJECTIVE: The mechanism underlying myometrial quiescence during pregnancy is unknown. Our group has previously shown that during pregnancy myometrial cyclic guanosine monophosphate content rises to several hundred times the nonpregnant levels, only to abruptly decline days before the onset of labor . Cyclic guanoeine monophosphate plays an integral role in the relaxation o f smooth muscle. The aim of this investigation was therefore to determine t he effects of pregnancy on both soluble and particulate guanylate cyclase e nzymatic activities and messenger ribonucleic acid expressions. STUDY DESIGN: Myometrium was obtained from randomly cycling adult nonpregna nt guinea pigs and near-term (50-60 days' gestation) pregnant guinea pigs o f similar chronologic age. Subcellular fractions were prepared by different ial ultracentrifugation. Guanylate cyclase activity was determined by the c onversion of guanosine triphosphate to cyclic guanosine monophosphate under basal or stimulated conditions in either the soluble guanylate cyclase or particulate guanylate cyclase fraction. A nitric oxide donor, S-nitroso-N-p enacillamine, was used to activate soluble guanylate cyclase (n = 16 animal s in each group). Several natriuretic peptides (atrial natriuretic peptide, brain natriuretic peptide, and C-type natriuretic peptide) and uroguanylin were used to stimulate the different particulate guanylate cyclase isoform s guanylate cyclase A, guanylate cyclase B, and guanylate cyclase C, respec tively, in pregnant (n = 8) and nonpregnant (n = 6) animals. Cyclic guanosi ne monophosphate content was measured by radioimmunoassay, and enzymatic ac tivity was expressed as picomoles of cyclic guanosine monophosphate per mil ligram of protein per minute, Total guanylate cyclase represented the sum o f soluble guanylate cyclase and particulate guanylate cyclase activities fo r a tissue. To investigate whether the observed changes in guanylate cyclas e activity were paralleled by changes in receptor expression, messenger rib onucleic acid levels of the genes for guanylate cyclase A and guanylate cyc lase B isoforms were quantified by ribonuclease protection assay (n = 5 ani mals in each group). RESULTS: Under basal conditions particulate guanylate cyclase represented 7 8% (nonpregnant state) to 88% during pregnancy) of the total guanylate cycl ase activity in the guinea pig myometrium, Pregnancy further reduced myomet rial soluble guanylate cyclase (both basal and stimulated by nitric oxide) relative to the nonpregnant state. Pregnancy selectively increased atrial n atriuretic peptide-stimulated particulate guanylate cyclase activity (attri buted to guanylate cyclase A), although it did not change basal myometrial particulate guanylate cyclase activity in general. Guanylate cyclase B (par ticulate guanylate cyclase stimulated by C-type natriuretic peptide) and gu anylate cyclase C (particulate guanylate cyclase stimulated by uroguanylin) activities were unaltered by pregnancy. The selective increase in responsi veness of particulate guanylate cyclase to atrial natriuretic peptide durin g pregnancy was not paralleled by an increased in level of messenger ribonu cleic acid for the gene for guanylate cyclase A. CONCLUSION: Pregnancy reduced the in vitro responsiveness of the myometrial soluble guanylate cyclase to nitric bride while increasing the responsiven ess of the particulate isoform to atrial natriuretic peptide and brain natr iuretic peptide through a mechanism independent of any change in receptor e xpression.