Thyrotropin- and prostaglandin E 2-responsive adenyl cyclase in thyroid plasma membranes

Abstract

We studied TSH and PGE 2 effects on adenyl cyclase in bovine thyroid plasma membranes. With [α- 32P]ATP as substrate, there was no PGE 2 effect on adenyl cyclase in plasma membranes highly responsive to TSH and NaF (7- to 12-fold and 12- to 15-fold increase over control, respectively). Since enhanced plasma membrane Na +-K +-activated ATPase activity (40-fold greater than original homogenate) might have resulted in significant ATP hydrolysis despite the presence of an ATP-regenerating system, we repeated these studies with 32P-labeled 5′-adenylyl-imidodiphosphate ([ 32P]AMP-PNP), an ATP analogue which is an ATPase-resistant substrate for adenyl cyclase. With AMP-PNP as substrate, adenyl cyclase in all plasma membranes tested showed a dose-related response (133% to 226% increase) to both TSH (5 to 250 mU/ml) and PGE 2 (10 −6 to 10 −4 M), which response was inhibited, in each instance, by the prostaglandin antagonist 7-oxa-13-prostynoic acid (PY 1, 10 −5 M). Since guanyl nucleotides, e.g. GTP, have been shown to enhance basal and hormone-stimulated adenyl cyclase in liver plasma membrane, we examined the effect of 0.1 mM GTP on thyroid plasma membrane adenyl cyclase using [α- 32P]ATP as substrate. In the presence of GTP, both basal and TSH-stimulated adenyl cyclase were enhanced two fold; in addition, a stimulatory PGE 2 effect (two-fold increase) was reproducibly demonstrated. In contrast, the effect of fluoride ion was inhibited by GTP. PY 1 inhibited the stimulatory effects of both TSH and PGE 2 under these conditions. These findings suggest great similarity between TSH and PGE 2 adenyl cyclase receptor(s) in thyroid and are consistent with the premise that prostaglandins play an important role in TSH stimulation of thyroid function.