A derivative of amiloride blocks both the light-regulated and cyclic GMP-regulated conductances in rod photoreceptors.

Abstract

Vertebrate rod photoreceptors in the dark maintain an inward current across the outer segment membrane. The photoresponse results from a light-induced suppression of this dark current. The light-regulated current is not sensitive to either tetrodotoxin or amiloride, potent blockers of Na+ channels. Here, we report that a derivative of amiloride, 3',4'-dichlorobenzamil (DCPA), completely suppresses the dark current and light response recorded from rod photoreceptors. DCPA also blocks a cyclic GMP-activated current in excised patches of rod plasma membrane and a cGMP-induced Ca++ flux from rod disk membranes. These results are consistent with the notion that the Ca++ flux mechanism in the disk membrane and the light-regulated conductance in the plasma membrane are identical. DCPA also inhibits the Na/Ca exchange mechanism in intact rods, but at a 5-10-fold-higher concentration than is required to block the cGMP-activated flux and current. The blocking action of DCPA in 10 nM Ca++ is different from that in 1 mM Ca++, which suggests either that the conductance state of the light-regulated channel may be modified in high and low concentrations of Ca++, or that there may be two ionic channels in the rod outer segment membrane.

Antagonists of the cGMP‐gated conductance of vertebrate rods block the photocurrent in scallop ciliary photoreceptors.

“…Thus, blockage of the photocurrent by intracellular L-cis-diltiazem exhibits the same voltage dependence as that observed with extracellular application. The amiloride derivative 3',4'-dichlorobenzamil applied extracellularly suppresses the light-sensitive current of amphibian rods at concentrations that are approximately one order of magnitude lower than those required by L-cis-diltiazem (Nicol et al 1987). Figure 7A illustrates t interrupted, the response regained its original amplitude.…”

“…The amiloride derivative 3',4'-dichlorobenzamil proved to be a more powerful blocker, with a K% in the vicinity of 5/M. This substance has been reported to inhibit the Nat-Ca2P exchanger of secretory cells (Kaczorowski et al 1985) and vertebrate rods (Nicol et al 1987), as well as L-type voltagedependent Ca!+ channels (Suarez-Kurtz & Kaczorowski, 1988;Garcia et al 1990); however, the half-maximal concentration for blockage of the light-sensitive, cGMP-gated conductance of amphibian and of scallop photoreceptors is approximately 10-fold lower, suggesting a high specificity of action. Amiloride was also capable of antagonizing the photoresponse but with a potency approximately 100-fold lower than its derivative.…”

Odorant response of isolated olfactory receptor cells is blocked by amiloride

“…The dark-current of vertebrate photoreceptor cells (rod outer segments) is blocked by the amiloride analog DCPA (apparent inhibition constant 1 /xM). However, this compound appears to block the cGMPactivated channels from the cytosolic side of the membrane [51]. It is not known whether the darkcurrent is affected by amiloride itself.…”

“…For instance, the Na/Ca-exchanger is blocked with an apparent inhibition constant of 1-2 mM [34]. However, the sidechain modified analog 3',4'-dichlorobenzamil (DCPA) inhibits Na/Ca-exchange with an apparent inhibition constant in the range 15-50/xM [34,51].…”

Na+/Ca2+-K+Exchangers (NCKX):Functional Properties and Physiological Roles

“…Very little is known about the pharmacology of NCKX proteins. Earlier work on NCKX1 in retinal rod outer segments showed that 3',5' dichlorobenzamil, 36 tetracaine and L-cis diltiazem 4 could block forward exchange at relatively high concentrations, while the same compounds were much more effective in inhibiting cGMP-gated channels present in this preparation. This implies that these compounds are unlikely to serve as useful diagnostics to detect NCKX activity.…”