Effect of colchicine and taxol on stimulation of G protein GTPase activity in anterior pituitary lobe of rats by gonadotrophin- and thyrotrophin-releasing hormones.

Colchicine and taxol, which are known to influence tubulin function, were used to delineate the possible role of tubulin in signal transduction in the anterior pituitary lobe. Anterior pituitary lobes, obtained from adult male rats, were processed by discontinuous sucrose gradient centrifugation to obtain plasma membranes. The low Km GTPase activity (EC 3.6.1.) was assayed in 5 micrograms membrane protein using [gamma-32P]GTP at 37 degrees C in an ATP-regenerating buffer containing 1 mumol unlabelled GTP l-1. Ten nmol l-1 each of colchicine, lumicolchicine and taxol maximally stimulated the GTPase activity by about 40% (P < 0.05). A time-course study revealed that 100 nmol colchicine l-1 stimulated the enzyme activity by 55, 74, 89 and 53% at 5, 10, 20 and 30 min, respectively (P < 0.05); lumicolchicine (100 nmol l-1) stimulated the GTPase activity by 44, 36, 11 and 55% at 5, 10, 20 and 30 min, respectively (P < 0.05). Taxol (100 nmol l-1) stimulated the enzyme activity by 39 and 25% at 20 and 30 min, respectively (P < 0.05). Gonadotrophin-releasing hormone (GnRH) and thyrotrophin-releasing hormone (TRH) stimulated the low Km GTPase activity in a concentration-dependent manner, by up to 40-60% (P < 0.05). In the presence of 100 nmol colchicine l-1, the ability of GnRH or TRH to stimulate the GTPase activity was inhibited.(ABSTRACT TRUNCATED AT 250 WORDS)