MYOMETRIUM OF THE SPINY DOGFISH SQUALUS-ACANTHIAS - PEPTIDE AND STEROID REGULATION

The phylogenetic age of endocrine control of viviparous reproduction i n 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 alter ing the intensity or duration of contractions. In contrast, neurointer mediate lobe extract provoked a marked and reversible enhancement of t he duration and intensity of contractions but was ineffective in alter ing the frequency of contractions. In steroid-primed animals, untreate d and estradiol-17 beta (E(2))-treated animals exhibited a decrease in the frequency of activity after injection of sRLX in vivo while pretr eatment with progesterone (P-4) alone or in combination with E(2) full y suppressed the effects of sRLX. These results suggest that homologou s sRLX slows the frequency of spontaneous uterine contractions in thir d-trimester sharks (stage C) in which endogenous P-4 is reduced and E( 2) levels are rising. These data demonstrate the physiological importa nce of these hormones and the antiquity of reproductive tract control mechanisms.