Hibernation-induction trigger. II. In vitro effects of prairie dog plasma albumin on mouse vas deferens contractility

Involvement of opioid molecules in hibernation is well established, with the delta opioid receptor implicated in hibernation induction. Previous studies have shown that plasma albumin fractions (PAFs) from hibernating mammals contain an uncharacterized ligand called "hibernation-induction trigger" (HIT), which causes inhibition of induced contractility in the guinea pig ileum (GPI). In part I of this study, we described effects of PAF from two species of prairie dogs on induced contractility of the GPI. In the present study (part II), we examine the response of the mouse vas deferens (MVD), which is populated with the delta receptor subtype, to increasing concentrations of PAF from the white-tailed prairie dog (WT) and the black-tailed prairie dog (BT). Dose-response curves of lyophilized PAF yielded IC50 values (mg) (mean dose that inhibits contractility to 50% of control) of 11.0 for summer WT, 10.6 for hibernating WT, 9.4 for summer BT, 12.2 for winter active BT, and 4.7 for winter hibernating BT. These results suggest that delta opioid (HIT) is present in both species throughout the calendar year and that the induction of hibernation may involve not only levels of opioid but also dynamic interactions between endogenous opioid and its receptors.