Neuronal Ca(V)1.3alpha(1) L-type calcium channels: Molecular, functional, and pharmacological diversity

Molecular and functional diversity among voltage-gated calcium channels underlies their crucial role in regulating many different signaling pathways in neuronal and other excitable cells. L-type calcium channels alone regulate a diverse array of cellular functions including excitation-contraction, excitation-secretion coupling, and gene expression. Of the four molecularly defined subclasses of L-type Ca channels, two are expressed ubiquitously in the mammalian nervous system (Ca_v1.2α₁ and Ca_v1.3α ₁). Despite diversity at the molecular level, neuronal L-type channels are generally assumed to be functionally and pharmacologically similar, i.e. high-voltage activated and highly sensitive to dihydropyridines. The Ca _v1.3α₁ gene undergoes extensive alternative splicing, that will generate multiple mRNA products, and consequently multiple proteins with small sequence variations. I cloned Ca_v1.3α₁ cDNAs from rat sympathetic neurons. When expressed in heterologous systems Xenopus oocytes and mammalian tsA-201 cells, Ca_v1.3α ₁ L-type channels had characteristics distinct from a typical L-type calcium channel. Ca_v1.3α₁ L-type channels activate at membrane potentials ∼25 mV more hyperpolarized compared to Ca _v1.2α₁, independent of the specific auxiliary subunits co-expressed, of alternative splicing in domains I–II, IVS3–IVS4, and the C-terminus, and of the expression system. Ca_v1.3α ₁ L-type channels also activated with a faster time course compared to Ca_v1.2α₁ L-type calcium channels. This relatively fast activation suggests that Ca_v1.3α₁ channels can carry significant Ca influx during an action potential-waveform. Ca _v1.3α₁ channels exhibit strong Ca-dependent inactivation. These features may be important for preventing prolonged Ca influx through these channels at hyperpolarized membrane potentials. In addition to having unusual functional properties, Ca_v1.3α₁ L-type channels are pharmacologically distinct from Ca_v1.2α ₁. The IC₅₀ for nimodipine block of Ca_v1.3α ₁ L-type calcium channel currents is 2.7 ± 0.3 μM, a value 20-fold higher than the concentration required to block Ca_v1.2α ₁. The relatively low sensitivity of the Ca_v1.3α ₁ subunit to inhibition by dihydropyridine is independent of alternative splicing in the IVS3–IVS4 linker. Collectively, our results suggest that functional and pharmacological criteria used commonly to distinguish among different Ca currents, will greatly underestimate the contribution of L-type channels in cells expressing Ca_v1.3α₁. The unique properties of Ca_v1.3α₁ L channels suggest that Ca_v1.3α₁ channels contribute to dendritic active membrane properties and neuron excitability.