EFFECTS OF CALCIUM-CHANNEL BLOCKERS ON RABBIT CORNEAL ENDOTHELIAL FUNCTION

The effects of calcium channel antagonists and agents that alter intracellular Ca2+ mobilization on corneal endothelial function have been examined. All experiments, except where specifically designated, were performed in the continuous presence of extracellular Ca2+. Verapamil (at 50 mu M) increased the swelling rate of corneas bathed in normal Ringer solution whereas nifedipine and diltiazem (both up to 100 mu M) were without effect. The nifedipine analog nisoldipine caused corneal swelling at 10 mu M and 50 mu M but nimodipine was without effect. When briefly exposed to a Ca2+-free solution corneal swelling was enhanced after subsequent exposure to 50 mu M verapamil in normal Ringer but not after 50 mu M diltiazem in normal Ringer, indicating that Ca2+ entry from the bathing solution into the cell was important and was apparently impeded by verapamil. Cadmium (0.6 and 1 mM) but not nickel (up to 250 mu M) caused swelling of corneas bathed in normal Ringer, A Ca2+ channel agonist, BAY-K-8644, alone did not influence corneal thickness but when presented to the endothelium with 50 mu M verapamil the swelling rate was much reduced compared to verapamil alone. The agonist, therefore, presumably maintained some Ca2+ channels open in face of the Ca2+ channel blocker. An agent that inhibited the release of intracellular Ca2+ stores (TMB-8) caused an initial corneal swelling over the first 1.5 hr of perfusion but thereafter had no effect on corneal thickness. In the presence of continued extracellular Ca2+ one explanation for the results is that modulation of intracellular Ca2+ by agents that alter plasma membrane transfer of Ca2+ influences apical junction permeability. This was confirmed by direct measurements of inulin and dextran permeability across the endothelium; nisoldipine, verapamil and Cd2+, but not Ni2+, increased permeability in the presence of extracellular Ca2+, a result that correlated with the increased swelling rate caused by nisoldipine, verapamil and Cd2+, while Ni2+ had no effect on corneal thickness. Nimodipine departed from this scheme by not having any effect on corneal thickness but causing a significant increase in non-electrolyte permeability. The results, collectively, thus support the notion of plasma membrane-located Ca2+ channels, probably of the L-type, that regulate the movement of this ion and influence normal physiological corneal endothelial function.