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Publisher Summary This chapter presents evidence for the contractile properties of the trabecular meshwork (TM) and their effects on aqueous humor reabsorption. In a study discussed in the chapter, membrane voltage measurements and patch-clamp techniques were applied to cultured bovine and human TM and ciliary muscle (CM) cells. Measurements of isometric tension were performed on isolated TM and CM strips. Anterior segments of bovine eyes with well-preserved TM were perfused to measure the outflow rate. Cultured bovine and human TM cells showed voltage spikes typical of smooth muscle cells that were inhibited by nifedipine but insensitive to tetrodotoxin. The excitability of TM cells indicates that they function as contractile smooth muscle cells. There is no principal difference between human and bovine TM cells in terms of K + and Ca 2+ channels, functional receptors for endothelin, and the effects of cholinergic and adrenergic agonists. The concept of functional antagonism between TM and CM should be considered in the interpretation of the mechanism of action of currently used antiglaucoma drugs and for the search for effective new drugs.

The regulation of intracellular pH (pHi) was monitored in a virus-transformed cell clone derived from bovine ciliary body exhibiting characteristics of pigmented ciliary epithelium. Data were obtained from confluent monolayers grown on plastic coverslips in nominally bicarbonate-free media using the pH-sensitive absorbance of 5- (and 6-) carboxy-4',5'-dimethylfluorescein. Under resting conditions, pHi averaged 6.98 +/- 0.01 (SEM; n = 57). When cells were acid loaded by briefly exposing them to Ringer containing NH4+ and then withdrawing the NH4+, pHi spontaneously regained its initial value. In the presence of 1 mM amiloride or in the absence of Na+, this process was blocked, indicating the involvement of an Na+/H+ exchanger in the regulation of pHi after an acid load. Removing Na+ during resting conditions decreased cytoplasmatic pH. This acidification could be slowed by amiloride, which is evidence for reversal of the Na+/H+ countertransport exchanging intracellular Na+ for extracellular protons. Application of 1 mM amiloride during steady state led to a slow acidification. Thus the Na+/H+ exchanger is operative during resting conditions extruding protons, derived from cellular metabolism, or from downhill leakage into the cell. Addition of Na+ to Na+ -depleted cells led to an alkalinization, which was sensitive to amiloride, with an IC50 of about 20 microM. This alkalinization was attributed to the Na+/H+ exchanger and exhibited saturation kinetics with increasing Na+ concentrations, with an apparent KM of 29.6 mM Na+. It is concluded that Na+/H+ exchange regulates pHi during steady state and after an acid load.