Ali Hassan

A. M. A. Hassan & D.R. Mason

Department of Zoology, University of Canterbury, Christchurch, New Zealand.

Recently, we have shown that treatment of ovine anterior pituitary cells with AVP, an important regulator of ACTH secretion, results in a desensitization to a subsequent stimulation with AVP. This desensitization was found to be rapid and readily reversible, characteristics which suggested that the desensitization may be mediated by phosphorylation of the pituitary AVP receptor, a mechanism of desensitization common amongst the G protein-coupled receptors (GPCRs). The aim of this study was to investigate the involvement of protein kinase C (PKC) and casein kinase 1α (CK1α), two kinases known to be involved in the phosphorylation of GPCRs, in this desensitization. Perifused anterior pituitary cells were treated with 5 min 100 nM AVP pulses after 100, 180 and 260 min, resulting in three similar peaks of ACTH secretion. Pre-treatment with AVP prior to the second pulse resulted in a reduction in response, or desensitization, the extent of which was assessed by expressing the response to the second pulse as a percentage of the mean of the responses to the first and third pulses. Pre-treatment with 10 nM AVP for 15 min reduced the response to the second pulse by 48.6 ± 1.6% (n=6, p<0.0001). Conversely, pre-treatment with the specific PKC activator 1,2-dioctanoyl-sn-glycerol did not cause a reduction in response–in fact there was a 51.3 ± 4.4% (n=6, p<0.0001) increase in ACTH secretion following pre-treatment with 300 µM DiC₈ for 15 min. Also, pre-treatment with 10 nM AVP for 15 min in combination with 2 µM Ro 31-8220, a specific PKC inhibitor, did not alter the magnitude of desensitization observed compared with controls which were pre-treated with 10 nM AVP alone. Similarly, pre-treatment for 15 min with the specific CK1α inhibitor CK1-7 at concentrations up to 100 µM in combination with 10 nM AVP had no effect on the magnitude of desensitization observed. Overall, these results do not support the involvement of either PKC or CK1α in the desensitization of the ACTH response to AVP.

Presented at the 11th International Congress of Endocrinology, Sydney, Australia, Abstract P1406, 2000.