Regional differences in the expression of tetrodotoxin-sensitive inward Ca2+ and outward Cs+/K+ currents in mouse and human ventricles

Low voltage-activated channels in rat pulmonary vein cardiomyocytes: coexistence of a non-selective cationic channel and of T-type Ca channels

In rat pulmonary vein (PV) cardiomyocytes (CM), norepinephrine (NE) induces an automatic activity consisting of bursts of slow action potentials which depend on Ca²⁺ (upstroke) and Na⁺ (inter-burst) channels. Our objective was to characterize low voltage-activated (LVA) currents in rat PVCM susceptible to trigger this activity. Whole-cell I_Ca (5 mM Ca²⁺) was recorded from - 100 mV with classical Na⁺- and K⁺-free solutions. A fast LVA I_Ca (FLVA-I_Ca), present in ≈ 56% of PVCM between ~ - 50 to - 20 mV, was blocked by 10 μM TTX and markedly increased by addition of NaCl (1 or 3 mM) or KCl (5 or 10 mM). Permeability ratios P'_Ca/P_Na and P'_Ca/P_K calculated for bi-ionic conditions were respectively 2.25 ± 0.51 and 1.88 ± 0.25, and not different from a value of 2. FLVA-I_Ca was increased by 10 μM NE and 300 nM BayK8644, decreased by 5 μM nifedipine but not blocked by ranolazine (10 μM). NiCl₂ (40 μM) and TTA-A2 (10 or 100 nM) increased FLVA-I_Ca. Similar results were obtained in left atrial (LA) CM. Neither Ba²⁺ nor Sr²⁺ alone could permeate the FLVA channel or block Ca²⁺ influx but revealed a large slower activating and inactivating LVA Ca²⁺ current (SLVA-I_Ca), present in 10 out of 80 PVCM, absent in LACM, and partially inhibited by 100 nM TTA-A2. Therefore, the ionic channel underlying FLVA-I_Ca is likely a fast voltage-gated non-selective channel with a dihydropyridine binding site. SLVA-I_Ca might correspond to Ca²⁺ influx through Ca_v3.x channels and contribute to triggering NE-induced automatic activity in the PV myocardial sleeve.