The dihydropyridine-sensitive calcium channel subtype in cone photoreceptors.
High-voltage activated Ca channels in tiger salamander cone photoreceptors were studied with nystatin-permeabilized patch recordings in 3 mM Ca2+ and 10 mM Ba2+. The majority of Ca channel current was dihydropyridine sensitive, suggesting a preponderance of L-type Ca channels. However, voltage-dependent, incomplete block (maximum 60%) by nifedipine (0.1-100 microM) was evident in recordings of cones in tissue slice. In isolated cones, where the block was more potent, nifedipine (0.1-10 microM) or nisoldipine (0.5-5 microM) still failed to eliminate completely the Ca channel current. Nisoldipine was equally effective in blocking Ca channel current elicited in the presence of 10 mM Ba2+ (76% block) or 3 mM Ca2+ (88% block). 15% of the Ba2+ current was reversibly blocked by omega-conotoxin GVIA (1 microM). After enhancement with 1 microM Bay K 8644, omega-conotoxin GVIA blocked a greater proportion (22%) of Ba2+ current than in control. After achieving partial block of the Ba2+ current with nifedipine, concomitant application of omega-conotoxin GVIA produced no further block. The P-type Ca channel blocker, omega-agatoxin IVA (200 nM), had variable and insignificant effects. The current persisting in the presence of these blockers could be eliminated with Cd2+ (100 microM). These results indicate that photoreceptors express an L-type Ca channel having a distinguishing pharmacological profile similar to the alpha 1D Ca channel subtype. The presence of additional Ca channel subtypes, resistant to the widely used L-, N-, and P-type Ca channel blockers, cannot, however, be ruled out.
Wilkinson
M F
MF
Neuroscience Research Group, University of Calgary, Alberta, Canada. wilkinso@acs.ucalgary.ca
Barnes
S
S
D000557:Ambystoma; D000818:Animals; D001464:Barium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism*; D004095:Dihydropyridines / Q000494:pharmacology*; D004594:Electrophysiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology; D007700:Kinetics; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018408:Patch-Clamp Techniques; D010455:Peptides / Q000494:pharmacology; D010786:Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D017949:Retinal Cone Photoreceptor Cells / Q000187:drug effects / Q000378:metabolism*; D013111:Spider Venoms / Q000494:pharmacology; D013569:Synapses / Q000187:drug effects / Q000502:physiology; D020867:omega-Agatoxin IVA; D020866:omega-Conotoxin GVIA
D002121:Calcium Channel Blockers; D015220:Calcium Channels; D004095:Dihydropyridines; D010455:Peptides; D013111:Spider Venoms; D020867:omega-Agatoxin IVA; D001464:Barium; D020866:omega-Conotoxin GVIA