ALTERED REGULATION OF L-TYPE CHANNELS BY PROTEIN-KINASE-C AND PROTEIN-TYROSINE KINASES AS A PATHOPHYSIOLOGIC EFFECT IN RETINAL DEGENERATION

The effect of protein tyrosine kinases (PTK) on L-type calcium channel s in cultured retinal pigmented epithelium (RPE) from rats with retina l dystrophy was investigated. Barium currents through Bay K 8644 (10(- 6) M) sensitive L-type channels were measured using the patch-clamp te chnique. The current density of L-type currents is twice as high and t he inactivation time constants are much slower than in cells from nond ystrophic control rats. Application of the PTK blockers genistein, lav endustin A, and herbimycin A (all 5X10(-6) M) led to an increase of Lr type currents. Intracellular application of pp60c-src (30 U/ml) via t he patch pipette led to a transient decrease of L-type currents. The p rotein kinase A (PKA) and PKG blocker H9 (10(-6) M) showed no effect o n L-type currents, However, the protein kinase C blocker chelerythrine (10(-5) M) reduced these currents. Up-regulation of PKC by 10(-6) M 4 beta-phorbol- 12 myristate-13-acetate (PMA) led to a decrease of C ty pe currents. Additional application of genistein led to a further decr ease of these currents. However, intracellular application of pp60(c-s rc) in PMA-treated cells led to a transient increase of L-type current s. Investigating the calcium response to bFGF application showed that RPE cells from RCS rats used different pathways than control RPE cells to increase cytosolic free calcium. This different pathway does not i nvolve the activation of L-type channels. The present study with RPE c ells from rats with retinal dystrophy shows a changed integration of P TK and PKC in channel regulation, Considering the altered response to bFGF in RCS-RPE cells, this disturbed regulation of L-type channels by tyrosine kinases is involved in the etiology of retinal degeneration in RCS rats.