Voltage-dependent calcium channels in the rat retina: involvement in NMDA-stimulated influx of calcium.
Rises in intracellular Ca2+ induced by activation of glutamate receptors are of ultimate importance for neuronal excitability and pathophysiological processes. In the present study, we aimed to elucidate the types of voltage-dependent Ca2+ channels involved in the NMDA-stimulated influx of Ca2+ into the isolated rat retina by using selective blockers. Additionally, the number of binding sites for radioligands labelling L- ([3H]nitrendipine), N- ([125I]omega-conotoxin MVIIA) and P/Q-type ([125I]omega-conotoxin MVIIC) Ca2+ channels was assessed in the rat retina and, for further comparison, in the rat cortex. Incubation of isolated rat retinas with 100 microM NMDA produced a three-fold increase in the influx of 45Ca2+ that was completely blunted by MK-801, a NMDA receptor antagonist, and partially attenuated (approximately 20%) by tetrodotoxin, a Na+ channel blocker. The L-type Ca2+ channel blocker nifedipine reduced NMDA-stimulated Ca2+ influx in a dose-related fashion, with a maximum reduction of approximately 50%. Similar effects were observed with verapamil and diltiazem. Blockers of N- and P/Q-type Ca2+ channels had no significant effect on the influx of Ca2+ evoked by NMDA. Co2+, a non-specific Ca2+ channel blocker, caused an inhibition of NMDA-stimulated Ca2+ influx similar to that of nifedipine. Therefore, of all voltage-dependent Ca2+ channels, L-type channels appear to make the greatest contribution (up to 50%) to the NMDA-stimulated influx of Ca2+ into the isolated rat retina. This finding contrasts with evidence obtained in brain neurones supporting a role for L-, N- and P/Q-type channels in NMDA-evoked Ca2+ signals. A comparison of the number of radioligand binding sites associated with L-, N- or P/Q-type Ca2+ channels in the rat cortex and retina revealed that such a difference cannot be ascribed to a distinct expression pattern of these channels in both tissues, although some variations were found. Interestingly, a different affinity of [3H]nitrendipine for L-type Ca2+ channels in the rat retina and cortex was observed which may reflect the expression of different classes of L-type channels in these tissues. The ability of L-type Ca2+ channel blockers to attenuate NMDA-stimulated Ca2+ influx may underlie their neuroprotective effects in the retina.
Nuffield Laboratory of Ophthalmology, University of Oxford, Walton Street, Oxford, OX2 6AW, UK.
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Melena
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Osborne
D000704:Analysis of Variance; D000818:Animals; D002121:Calcium Channel Blockers / Q000494:pharmacology; D020746:Calcium Channels, L-Type / Q000502:physiology*; D020864:Calcium Channels, N-Type / Q000502:physiology*; D020865:Calcium Channels, P-Type / Q000502:physiology*; D020907:Calcium Channels, Q-Type / Q000502:physiology*; D004110:Diltiazem / Q000494:pharmacology; D016291:Dizocilpine Maleate / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D016202:N-Methylaspartate / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D011869:Radioligand Assay; D051381:Rats; D017208:Rats, Wistar; D016194:Receptors, N-Methyl-D-Aspartate / Q000037:antagonists & inhibitors; D012044:Regression Analysis; D012160:Retina / Q000502:physiology*; D013779:Tetrodotoxin / Q000494:pharmacology; D014700:Verapamil / Q000494:pharmacology
D002121:Calcium Channel Blockers; D020746:Calcium Channels, L-Type; D020864:Calcium Channels, N-Type; D020865:Calcium Channels, P-Type; D020907:Calcium Channels, Q-Type; D016194:Receptors, N-Methyl-D-Aspartate; D013779:Tetrodotoxin; D016202:N-Methylaspartate; D016291:Dizocilpine Maleate; D014700:Verapamil; D004110:Diltiazem; D009543:Nifedipine