Endocrine Abstracts

Natriuretic peptides, ANP, BNP and CNP, exert the majority of their effects via particulate guanylyl cyclase receptors (GC-A or GC-B), resulting in increased cellular cGMP levels. Several studies have examined the pharmacological properties of GC-B receptors in many systems, and desensitization (tachyphylaxis, loss of response) has been reported to occur. Targeted deletion (in mice) or genetic mutations (in humans) of the GC-B receptor have been implicated in a dwarfism phenotype, potentially due to a combined growth hormone deficiency as well as achondraplasia, suggesting that somatotrophs are the predominant pituitary target of CNP. However, despite this intriguing molecular data, the pharmacology of somatotroph guanylyl cyclases has yet to be fully elucidated. Using cGMP-enzymeimmunoassays, GH3 cells were shown to respond to a range of concentrations of ANP and CNP in the presence of 1mM IBMX, but not to the nitric oxide donor, sodium nitroprusside. To establish whether the GC-B receptor rapidly desensitized in GH3 cells, short-time course and pre-treatment experiments were performed. Rapid CNP-stimulated cGMP accumulation occurred between 0 and 5 min (r²=0.85,****), followed by apparent desensitisation between 5–15 min (r²=0.12, ns). Pre-treatment with CNP for up to 6 h also caused a significant reduction in the ability of CNP to subsequently stimulate cGMP accumulation (by 41.6±2.7%,**). This effect was receptor specific, because pre-treatment experiments with ANP did not cause desensitisation of CNP-stimulated cGMP accumulation. Finally, as sphingosine-1-phosphate (S1P) was recently shown to cause GC-B desensitization in fibroblasts, we examined the effect of S1P on CNP signalling in GH3 cells. S1P pre-treatment (1 h, 10 μM) caused a significant inhibition in CNP-stimulated cGMP accumulation (to 68.9±2.1% of control,**). These data reveal multiple mechanisms of GC-B regulation exist in GH3 somatotrophs, which may assist the design of therapeutic treatments associated with CNP or GC-B deficiency. Funded by a BBSRC Project Grant (BBD0015601).