Analysis of vasoactive intestinal peptide receptors and the G protein regulation of adenylyl cyclase in seminal vesicle membranes from streptozotocin-diabetic rats

Abstract

The present report describes the status of the vasoactive intestinal peptide (VIP) receptor/effector system of signal transduction in seminal vesicle from streptozotocin (STZ)-treated rats. STZ-treatment modified the binding parameters of the high-affinity sites for VIP in seminal vesicle: 0.78 ± 0.10 and 2.54 ± 0.30 nM for the dissociation constant ( K d) in control and diabetic rats, respectively; 0.07 ± 0.01 and 0.15 ± 0.03 pmol VIP/mg protein for the maximum binding capacity ( B max) in control and diabetic rats, respectively. It was associated with a reduced potency of VIP on the stimulation of adenylyl cyclase activity in the diabetic state ( ed 50 = 64.0 ± 20.0 nM ) as compared to control ( ed 50 - 9.5 ± 4.3 nM ). In contrast, the stimulatory effects of GTP, Gpp[NH]p and forskolin on the enzyme activity were not modified in diabetic rats. The levels of G-protein subunits in rat seminal vescicle were studied by immunoblot of α s and α i subunits: whereas α 1-subunit levels did not vary, those corresponding to α s subunit decreased after STZ treatment. In diabetic rats, low concentrations of Gpp[NH]p failed to inhibit forskolin-stimulated adenylyl cyclase activity, suggesting the absence of functional G i in this condition. In conclusion, present results show a decrease in the sensitivity of the VIP receptor/effector system in seminal vesicle membranes from STZ-treated rats suggesting a physiopathological role for VIP in the seminal neuropathy observed in diabetes.