Isolation and properties of sea turtle (Chelonia mydas) pituitary prolactin

The complete amino acid sequence of prolactin from the sea turtle (Chelonia mydas)

The complete amino acid sequence of prolactin (PRL) from a reptile, the sea turtle (Chelonia mydas), was determined for the first time. Sequence analysis was performed on fragments obtained from cleavage of intact and performic acid-oxidized hormone with lysyl endopeptidase, Staphylococcus aureus protease, and o-iodosobenzoic acid employing manual Edman degradation. The sea turtle PRL consists of 198 amino acid residues with three disulfide linkages formed between residues 4-11, 58-173, and 190-198 and possesses heterogeneity indicated by four replacements at positions 55, 145, 148, and 171. Sequence comparison with other vertebrate PRLs revealed that the degree of sequence identity conforms well to expectations based on phylogeny except for the rodent PRLs; sea turtle PRL has 86% identity with chicken PRL; 81% with horse, pig, and fin whale PRLs; 75-71% with cattle, sheep, and human PRLs; 60-56% with mouse and rat PRLs; and 35-31% with carp, salmon, and tilapia PRLs.

Presence of prolactin receptors in kidney and large intestine of the snake Ptyas mucosa

125I-labeled ovine prolactin was used to test for the presence of prolactin receptors in membranes prepared from tissues of the common rat snake Ptyas mucosa. High levels of specific binding were found in kidney and large intestine membranes prepared from snakes of both sexes. The binding was time, temperature, and protein dependent. The presence of Ca2+ in the incubation medium enhanced the binding, with optimal concentrations at 10-20 mM. Specificity of binding was established by employing different hormones as the displacing species in the radioreceptor assay. The results suggested the presence of lactogenic receptors in snake kidney and large intestine membranes. Scatchard analysis of the binding data indicated the presence of a single class of binding sites in snake kidney membranes with a dissociation constant of 0.83 nM. The present study is the first report of the presence of prolactin receptors in snake kidney and large intestine membranes.

Temperature dependence of in vitro pituitary, testis, and thyroid secretion in a turtle, Pseudemys scripta

In vitro culture was used to examine the direct actions of temperature at the level of pituitary hormone [luteinizing hormone (LH), thyrotropin (TSH), growth hormone (GH), prolactin (PRL)] responses to neuropeptides and two related peripheral endocrine responses [thyroid hormone (T4) and testicular androgen secretion] to pituitary hormones (TSH and gonadotropins) in a turtle, Pseudemys scripta. All these responses were fully suppressed at very low temperatures (5-6 degrees) and maximal near the species' preferred body temperature (28 degrees), but sensitivities differed markedly in intermediate ranges. At the pituitary level, the response of TSH, GH, and PRL to thyrotropin-releasing hormone (TRH) was considerably more temperature sensitive than the response of LH to gonadotropin-releasing hormone (GnRH) stimulation. TSH, GH, and PRL were unresponsive at 20 degrees or below, whereas LH secretion was stimulated almost equally between 12 and 28 degrees; the main effect of cooling on LH secretion was to reduce the duration of the response to GnRH. There was no clear effect of previous thermal history on temperature sensitivity of pituitary neuropeptide responsiveness although the general responsiveness of the gland was altered; however, these latter effects may also be related to variations in other factors such as photoperiod, season, and nutrition. Temperature sensitivities of the thyroid and testes also differed, but in the opposite way from the related pituitary cell types. Thyroid glands were relatively insensitive to temperature and responded to TSH between 12 and 32 degrees, with no difference between 20 and 28 degrees. In contrast, testicular androgen secretion showed an abrupt decline in gonadotropin responsiveness below 28 degrees; dose sensitivity, response rate, and maximal output were affected. Results were similar for sea turtle LH, snapping turtle LH, and ovine follicle-stimulating hormone. Thus, the temperature dependence of the two endocrine systems may have a different rate-limiting component.

Effects of thyrotropin-releasing hormone in vitro on thyrotropin and prolactin release from the turtle pituitary

Effects of synthetic thyrotropin-releasing hormone (TRH) on thyrotropin (TSH) and prolactin (PRL) release by hemipituitaries of adult turtles, Chrysemys picta, were studied in an in vitro superfusion system. Significant increases in the rates of secretion of both immuno-reactive TSH and PRL occurred at doses between 0.01 and 10 ng/ml TRH. TSH secretion increased acutely by two-, to sixfold over nonstimulated secretion levels; responses tended to decline after many hours of continual stimulation, but output remained elevated above baseline in most cases. PRL secretion increased, parallel to TSH secretion during TRH stimulation. No significant difference was found in secretion rates between males and females, and no clear relationship between TRH responsiveness and reproductive stage was evident. These data provide the first direct evidence for the stimulation of TSH secretion by TRH in a reptile and confirm earlier reports that TRH stimulates the release of PRL in the turtle. Although previous in vivo studies indicated that TSH secretion was not affected by TRH in turtles, the present data indicate that the dose sensitivity of the chelonian gland is comparable with that of mammalian and avian pituitaries. Evidence for the role of TRH in endogenous TSH regulation is still lacking in reptiles but the present data provide evidence for functional TRH receptors on the chelonian thyrotrope and, hence, argue against the hypothesis that TSH stimulating activity of TRH evolved relatively recently in association with endothermy.