—Voltage-activated Ca²⁺channels exist as multigene families that share common structural features. Different Ca²⁺channels are distinguished by their electrophysiology and pharmacology and can be classified as either low or high voltage–activated channels. Six α1 subunit genes cloned previously code for high voltage–activated Ca²⁺channels; therefore, we have used a database search strategy to identify new Ca²⁺channel genes, possibly including low voltage–activated (T-type) channels. A novel expressed sequence–tagged cDNA clone of α1G was used to screen a cDNA library, and in the present study, we report the cloning of α1H (or Ca_vT.2), a low voltage–activated Ca²⁺channel from human heart. Northern blots of human mRNA detected more α1H expression in peripheral tissues, such as kidney and heart, than in brain. We mapped the gene,CACNA1H, to human chromosome 16p13.3 and mouse chromosome 17. Expression of α1H in HEK-293 cells resulted in Ca²⁺channel currents displaying voltage dependence, kinetics, and unitary conductance characteristic of native T-type Ca²⁺channels. The α1H channel is sensitive to mibefradil, a nondihydropyridine Ca²⁺channel blocker, with an IC₅₀of 1.4 μmol/L, consistent with the reported potency of mibefradil for T-type Ca²⁺channels. Together with α1G, a rat brain T-type Ca²⁺channel also cloned in our laboratory, these genes define a unique family of Ca²⁺channels.

β-adrenergic receptor (βAR) activation promotes relaxation of both vascular and airway smooth muscle cells (VSMCs and ASMCs, respectively), though the signaling mechanisms have not been fully elucidated. We previously found that the activity of Kv7.5 voltage-activated potassium channels in VSMCs is robustly enhanced by activation of βARs via a mechanism involving protein kinase A (PKA)-dependent phosphorylation. We also found that enhancement of Kv7 channel activity in ASMCs promotes airway relaxation. Here we provide evidence that Kv7.5 channels are natively expressed in primary cultures of human ASMCs and that they conduct currents which are robustly enhanced in response to activation of the βAR/cyclic adenosine monophosphate (cAMP)/PKA pathway. MIT Scansite software analysis of putative PKA phosphorylation sites on Kv7.5 identified 8 candidate serine or threonine residues. Each residue was individually mutated to an alanine to prevent its phosphorylation and then tested for responses to βAR activation or to stimuli that elevate cAMP levels. Only the mutation of serine 53 (S53A), located on the amino terminus of Kv7.5, significantly reduced the increase in Kv7.5 current in response to these stimuli. A phospho-mimic mutation (S53D) exhibited characteristics of βAR-activated Kv7.5. Serine-to-alanine mutations of 6 putative PKA phosphorylation sites on the Kv7.5 C-terminus, individually or in combination, did not significantly reduce the enhancement of the currents in response to forskolin treatment (to elevate cAMP levels). We conclude that phosphorylation of S53 on the amino terminus of Kv7.5 is essential for PKA-dependent enhancement of channel activity in response to βAR activation in vascular and airway smooth muscle cells.