High-voltage-activated calcium channels in Müller cells acutely isolated from tiger salamander retina

Müller cells mediate retinal function by stabilizing the ionic environment and signal glial network activity via calcium waves. Using whole-cell patch clamp recording, we describe a high-voltage-activated, slowly inactivating Ca channel current in isolated salamander Müller cells that has unusual pharmacological properties. The Ca channel current has an activation midpoint of ∼-8 mV and an inactivation midpoint of ∼-26 mV in 10 mM Ba²⁺. The time constant for inactivation is ∼380 ms at potentials positive to zero. The current is blocked by Cd²⁺ with an EC ₅₀ of <100 nM. nisoldipine (10 μM) blocks ∼50%, while nifedipine (1 μM), diltiazem (20 μM), and verapamil (50 μM) each block one-third of the current. In contrast to its typical actions, BayK 8644 blocks the current by ∼ 25%. Blockers of other Ca channel subtypes were also tested: ω-agatoxin IVA (200 nM) blocked only 13% of the Ca channel current, while ω-conotoxin GVIA (1 μM) blocked 84% of the current. Immnohistochemistry supported the presence of α1A, α1B, α1C, and α1D Ca channel subunits. Mapping of dihydropyridine-binding sites with DM-BODIPY revealed a distribution of channels over the entire membrane of the Müller cell with a higher density at the apical region. Overall, these observations suggest either the presence of a mix of L- and N-type Ca channels or a single, unconventional HVA Ca channel subtype sharing L- and N-type Ca channel characteristics.