Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork

Prostanoids and cannabinoids have ocular hypotensive and neuroprotective properties. The effect of the prostanoid AH13205 (EP2), the thromboxane-mimetic U46619, the cannabinoid (CB) agonists WIN55212-2 and CP 55,940, endothelin-1 (ET-1) and 8-bromo-cAMP on the membrane currents of trabecular meshwork (TM) cells were measured using the patch-clamp technique and compared to their effects on TM contractility. Previous studies show relaxation of TM to AH 13205 and other substances that elevate cAMP, while U46619 and endothelin-1 contract TM. This study shows that after contraction (100%) with carbachol (10−6 m), the CB agonist CP 55,940 dose-dependently reduced contractility to 83±4% (n=9) (10−6 m) and 61±10%, (n=7) (10−5 m). In the presence of both the CB1 antagonist AM251 (10−6 m) and CP 55,940 (10−5 m), the contractile response to carbachol reached 84±3% (n=6) of the original level. In patch-clamp experiments, membrane permeable 8-bromo-cAMP (10−4 m) had no effect on currents of TM cells. In contrast, AH 13205 and two cannabinoids reversibly enhanced outward current through high-conductance Ca2+-activated K+ channels (BKCa, BK, maxi-K) to the following values (in % of the initial value at 100 mV): AH 13205 (10−5 m): 200±28% (n=6), CP 55,940 (10−6 m): 196±33% (n=7), CP 55,940 (10−5 m): 484±113% (n=7), WIN55212-2 (10−5 m): 205±41% (n=10). Iberiotoxin (10−7 m) completely blocked these responses. The current response to CP 55,940 (10−5 m) could be partially blocked by the CB1 antagonist AM251 (10−6 m). Conversely, the contractile agents in this study either caused a transient reduction in outward current (ET-1(5×10−8 m)) or had no effect (U46619 (10−6 m)). We conclude that stimulation of EP2 and CB1 receptors in TM is coupled to the activation of BKCa channels via a non-diffusible second messenger cascade. This effect may contribute to the relaxant activity of EP2 and CB1 agonists in isolated TM strips, modulating ocular outflow.