Myometrium of the spiny dogfish Squalus acanthias: peptide and steroid regulation.

The phylogenetic age of endocrine control of viviparous reproduction in vertebrates may be estimated by examination of elasmobranch models. We have shown in pregnant Squalus acanthias that Squalus relaxin (sRLX) significantly decreased the frequency of myometrial contractions in a dose-dependent reversible manner in vitro and in vivo, without altering the intensity or duration of contractions. In contrast, neurointermediate lobe extract provoked a marked and reversible enhancement of the duration and intensity of contractions but was ineffective in altering the frequency of contractions. In steroid-primed animals, untreated and estradiol-17 beta (E2)-treated animals exhibited a decrease in the frequency of activity after injection of sRLX in vivo while pretreatment with progesterone (P4) alone or in combination with E2 fully suppressed the effects of sRLX. These results suggest that homologous sRLX slows the frequency of spontaneous uterine contractions in third-trimester sharks (stage C) in which endogenous P4 is reduced and E2 levels are rising. These data demonstrate the physiological importance of these hormones and the antiquity of reproductive tract control mechanisms.