Beta-adrenergic inhibition of rabbit lens anterior-surface K⁺ conductance

Purpose. To characterize the effects of cAMP-elevating stimuli on the rabbit translens electrical parameters and examine the distribution of beta adrenoceptors about the epithelial surface. Methods. The electrophysiological experiments encompassed the isolation of lenses within a vertically arranged, Ussing-type chamber under short-circuit conditions, an approach that allowed for measurements of short-circuit current (I_sc) across, in separate experiments, discrete surface regions. Epithelial beta receptors were localized by immunofluorescent labeling of lens cryosections primarily exposed to a polyclonal antibody against human β₂-adrenoceptors. Reverse transcription - polymerase chain reaction (RT-PCR) was used to generate cDNA (using specific primers based upon the sequence of the previously cloned human β₂ receptor) from rabbit lens RNA extracted from mechanically sequestered anterior and equatorial epithelial cells. Results. Asymmetrical I_sc reductions with increases in translens resistance were elicited with epinephrine, isoproterenol, terbutaline, forskolin, and a lipid-permeable cAMP analogue. Electrical changes were recorded across the anterior aspect and not observed when the above compounds were applied to solutions bathing the equatorial and posterior surfaces. Immunohistochemical observations indicated the expression of beta receptors from the anterior epithelium to the equatorial region. RT-PCR yielded cDNA of expected basepair length for the apparent fragment of the β₂-adrenoceptor, which exhibited a sequence homology 90% identical with its human equivalent in both the anterior and equatorial epithelia. Conclusions. The cAMP-sensitive conductance(s) appear limited to the anterior epithelium and undetectable equatorially. The asymmetrical I_sc responses do not seem to arise from a spatial heterogeneity in epithelial receptor expression.