AJP - Regulatory, Integrative and Comparative Physiology, Vol 269, Issue 2 389-R397, Copyright © 1995 by American Physiological Society
Myometrium of the spiny dogfish Squalus acanthias: peptide and steroid regulation
L. A. Sorbera and I. P. Callard
Department of Biology, Boston University, Massachusetts 02215, USA.
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.
