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

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 Ca2+ 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 Ca2+ imaging, we found that there is a Ca2(+) influx in proportion to I-h induced by hyperpolarization in HEK293 cells. The I-h channel blockers Cs+ and ZD7288 inhibit both HCN current and Ca2(+) influx. Measurements of the fractional Ca2+ 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 Ca2+-permeable AMPA-R (alpha-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 Ca2+ 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 Ca2+ influx at negative membrane potentials.