Calcium influx through hyperpolarization-activated cation channels (I_h channels) contributes to activity-evoked neuronal secretion

摘要

The hyperpolarization-activated cation channels (I_h) play a distinct role in rhythmic activities in a variety of tissues, including neurons and cardiac cells. In the present study, we investigated whether Ca~(2+) can permeate through the hyperpolarization-activated pacemaker channels (HCN) expressed in HEK293 cells and I_h channels in dorsal root ganglion (DRG) neurons. Using combined measurements of whole-cell currents and fura-2 Ca~(2+) imaging, we found that there is a Ca~(2+) influx in proportion to I_h induced by hyper-polarization in HEK293 cells. The I_h channel blockers Cs~+ and ZD7288 inhibit both HCN current and Ca~(2+) influx. Measurements of the fractional Ca~(2+) current showed that it constitutes 0.60 +- 0.02% of the net inward current through HCN4 at -120 mV. This fractional current is similar to that of the low Ca~(2+)-permeable AMPA-R (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor) channels in Purkinje neurons. In DRG neurons, activation of I_h for 30 s also resulted in a Ca~(2+) influx and an elevated action potential-induced secretion, as assayed by the increase in membrane capacitance. These results suggest a functional significance for I_h channels in modulating neuronal secretion by permitting Ca~(2+) influx at negative membrane potentials.