Voltage-gated channels and calcium homeostasis in mammalian rod photoreceptors.
Recent reports on rod photoreceptor neuroprotection by Ca2+ channel blockers have pointed out the need to assess the effect of these blockers on mammalian rods. However, in mammals, rod electrophysiological characterization has been hampered by the small size of these photoreceptors, which were instead extensively studied in nonmammalian vertebrates. To further characterize ionic conductances and to assess the pharmacology of Ca2+ channels in mammalian rods, freshly dissociated pig rod photoreceptors were recorded with the whole cell patch-clamp technique. Rod cells expressed 1) a hyperpolarization-activated inward-rectifying conductance (I(h)) sensitive to external Cs+; 2) a sustained outward K+ current (I(K)) sensitive to tetraethylammonium; 3) a sustained voltage-gated Ca2+ current (I(Ca)) sensitive to benzothiazepine (diltiazem) and phenylalkylamine (verapamil) derivatives; 4) a Ca(2+)-activated Cl- current (I(Cl(Ca))); and 5) a plasma membrane Ca(2+)-ATPase. The Ca2+ current showed a range of activation from positive potentials to -60 mV with a maximum between -30 and -20 mV. In contrast to other L-type Ca2+ channels, rod Ca2+ channels were blocked at similar and relatively high concentrations by the diltiazem isomers and verapamil. The biphasic dose-response for D-diltiazem confirmed the low sensitivity of Ca2+ channels for the molecule. The ATPase, which was localized at the axon terminal, was found to contribute to Ca2+ extrusion. These results suggest that the electrophysiological features of rod photoreceptors had been preserved during evolution from nonmammalian vertebrates to mammals. This work indicates further that mammalian rods express nonclassic L-type Ca2+ channels, showing a low sensitivity to the diltiazem isomers used in neuroprotective studies.
Cia
David
D
Laboratoire de Physiopathologie Cellulaire et Moléculaire de la Rétine, Institut National de la Santé et de la Recherche Médicale U-592, Université Pierre et Marie Curie.
Bordais
Agnès
A
Varela
Carolina
C
Forster
Valérie
V
Sahel
José A
JA
Rendon
Alvaro
A
Picaud
Serge
S
D000818:Animals; D000838:Anions / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002121:Calcium Channel Blockers / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D000252:Calcium-Transporting ATPases / Q000378:metabolism / Q000494:pharmacology; D027682:Cation Transport Proteins; D002412:Cations / Q000494:pharmacology; D002478:Cells, Cultured; D002586:Cesium / Q000494:pharmacology; D002712:Chlorides / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D004347:Drug Interactions; D004558:Electric Stimulation / Q000379:methods; D006706:Homeostasis / Q000502:physiology*; D007150:Immunohistochemistry / Q000379:methods; D015640:Ion Channel Gating / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D008564:Membrane Potentials / Q000502:physiology / Q000528:radiation effects; D018408:Patch-Clamp Techniques / Q000379:methods; D053499:Plasma Membrane Calcium-Transporting ATPases; D026902:Potassium Channel Blockers / Q000494:pharmacology; D017948:Retinal Rod Photoreceptor Cells / Q000187:drug effects / Q000502:physiology* / Q000528:radiation effects; D013552:Swine; D019789:Tetraethylammonium / Q000494:pharmacology
D000838:Anions; D002121:Calcium Channel Blockers; D015220:Calcium Channels; D027682:Cation Transport Proteins; D002412:Cations; D002712:Chlorides; D026902:Potassium Channel Blockers; D002586:Cesium; D019789:Tetraethylammonium; D053499:Plasma Membrane Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; C028019:cesium chloride; D002118:Calcium