Molecular and functional identification of cyclic AMP-sensitive BKCa potassium channels (ZERO variant) and L-type voltage-dependent calcium channels in single rat juxtaglomerular cells

This study aimed at identifying the type and functional significance of potassium channels and voltage-dependent calcium channels (Ca-v) in single rat JG cells using whole-cell patch clamp. Single JG cells displayed outward rectification at positive membrane potentials and limited net currents between - 60 and - 10 mV. Blockade of K+ channels with TEA inhibited 83% of the current at + 105 mV. Inhibition of K-V channels with 4-AP inhibited 21% of the current. Blockade of calcium-sensitive voltage-gated K+ channels (BKCa) with charybdotoxin or iberiotoxin inhibited 89% and 82% of the current, respectively. Double immunofluorescence confirmed the presence of BKCa and renin in the same cell. cAMP increased the outward current by 1.6-fold, and this was inhibited by 74% with iberiotoxin. Expression of the cAMP-sensitive splice variant (ZERO) of BKCa was confirmed in single-sampled JG cells by RT-PCR. The resting membrane potential of JG cells was - 32 mV and activation of BKCa with cAMP hyperpolarized cells on average 16 mV, and inhibition with TEA depolarized cells by 17 mV. The cells displayed typical high-voltage activated calcium currents sensitive to the L-type Ca-v blocker calciseptine. RT-PCR analysis and double-immunofluorescence labeling showed coexpression of renin and L-type Ca-v 1.2. The cAMP-mediated increase in exocytosis ( measured as membrane capacitance) was inhibited by depolarization to + 10 mV, and this inhibitory effect was blocked with calciseptine, whereas K+-blockers had no effect. We conclude that JG cells express functional cAMP-sensitive BKCa channels ( the ZERO splice variant) and voltage-dependent L-type Ca2+ channels.