The role of the sarcoplasmic reticulum in neonatal uterine smooth muscle: enhanced role compared to adult rat.

Little is known about contractile activity, response to agonists or excitation-contraction coupling in neonatal smooth muscle. We have therefore investigated 10-day rat uterus to better understand these processes, and compared it to adult uterus to elucidate how control of contractility develops. Spontaneous contractions are present in the 10-day neonatal uterus, although they are not as large or as regular as those present in adult tissues. External Ca(2+) entry via L-type Ca(2+) channels is the sole source of Ca(2+) and is essential for the spontaneous activity. The neonatal uterus was responsive to carbachol or prostaglandin F(2alpha) application; it showed a marked stimulation and a clear dissociation between the force and Ca(2+) changes. Such sensitization was not apparent in adult rat myometrium. The sarcoplasmic reticulum (SR) had more releasable Ca(2+) and contributed more to the response to agonists in neonatal compared to adult tissues. Thus, Ca(2+) entry as opposed to SR Ca(2+) release contributed much less to the uterine response to agonists in the neonatal, compared to adult tissues. Inhibition of the SR by cyclopiazonic acid also caused a more vigorous increase in Ca(2+) and contractile activity, particularly frequency, in the neonatal compared to the adult uterus. Taken together these data suggest that: (1) spontaneous activity is already present by day 10, (2) receptor-coupling and excitation-contraction signalling pathways are functional, (3) the SR and Ca(2+) sensitization mechanisms play a more prominent role in the neonate, and (4) there is a shift to a greater reliance on Ca(2+) entry and excitability with development of the myometrium.