353 CYCLIC AMP HYDROLYZING, CYCLIC-GMP INHIBITED PHOSPHODIESTERASE (PDE3B) IS HIGHLY EXPRESSED IN BOVINE AND GECKO PHOTORECEPTORS

A cGMP-specific phosphodiesterase (PDE6) preferentially hydrolyzes cGMP in the outer segments of photoreceptors. The level of cGMP controls the polarization state of photoreceptors through cGMP-gated channels in the disc membranes. Preliminary experiments suggest that PDE3B, a cAMP hydrolyzing, cGMP-inhibited PDE, may be found in photoreceptors PDE3B, a physiologically important mediator of insulin, IGF, and leptin, mediates an effect of cGMP on cAMP function in the smooth muscle. High expression of cGMP-inhibited PDE3B may mediate a similar process in photoreceptors by lowering the levels of intracellular cAMP, thus decreasing protein kinase A (PKA) activity. PKA phosphorylates phosducin, which when phosphorylated, does not bind to the beta-gamma subunit of transducin. Immunocytochemistry localize PDE3B to bovine retina with highest signal found over the cone outer segments, as well as to both the inner and outer segments of gecko rods. This was confirmed with western blots using specific antisera to the C-terminal 1/3 region of PDE3B, which was blocked by preincubation with PDE3B-GST fusion protein. There were no significant changes in the photoreceptor light response measured by single cell recordings of individual salamander rod and cone cells in the presence of 10μM milrinone (a PDE3 specific inhibitor). Intraperitoneal injections of 3mg/kg milrinone and 0.3mg/kg trequinsin (another more potent PDE3 inhibitor) did not significantly influence the A or B wave of mouse electroretinograms in response to flashes of light. These findings report a cAMP hydrolyzing, cGMP-inhibited PDE highly expressed in bovine and gecko photoreceptor cells. Under the conditions tested, PDE3B may exist in an inactive state in photoreceptors.