Ca²⁺ is a second messenger in olfactory receptor cells, important in their response and adaptation. Ca²⁺ enters through cyclic nucleotide-gated channels and is believed to leave via an exchange extrusion mechanism driven by the Na⁺ gradient (Reisert & Matthews, 1998). To further characterise Na⁺-Ca²⁺ exchange we have measured electrogenic currents associated with Ca²⁺ extrusion in isolated frog olfactory receptor cells.

Frogs, Rana temporaria, were killed by stunning then rostral and caudal pithing. The cell body of an isolated olfactory receptor cell was drawn into a suction pipette to measure currents flowing across the cilia, and the solution bathing the cilia rapidly exchanged by translating them between four flowing streams under computer control. To measure the electrogenic current associated with Na⁺-Ca²⁺ exchange, the cilia were translated from normal Ringer to a solution in which guanidinium was substituted for Na⁺ to incapacitate Ca²⁺ extrusion (Reisert & Matthews, 1998), which included 500 µM of the phosphodiesterase inhibitor IBMX to elevate the cAMP concentration within the cilia and allow loading with Ca²⁺ through the cyclic nucleotide gated channels. After 4 s, the cilia were translated for 2 s into a guanidinium-substituted solution without IBMX, to allow cAMP levels to fall and cyclic nucleotide-gated channels to close while maintaining Ca²⁺_i at an elevated level; this solution included 1 mM niflumic acid to block the Ca²⁺-activated Cl^– conductance. Then the cilia were returned to Ringer with 1 mM niflumic acid, allowing extrusion of the Ca²⁺ load. Junction currents between these solutions and Ringer in the suction pipette were corrected by subtraction of the currents obtained when the protocol was repeated in the absence of IBMX, thereby preventing Ca²⁺ loading.

Upon the return to Ringer after Ca²⁺ loading a decaying current was observed, which was not present in recordings without IBMX. In experiments for which the junction currents were stable throughout, the junction-corrected current could be fitted by a single exponential with a mean amplitude of 1.67 pA and a time constant of 1.22 ± 0.06 s (mean ± S.E.M.; n = 11 cells). The amplitude and time constant of this current were little affected by doubling the concentration of niflumic acid (2.09 pA, 1.56 ± 0.11 s; n = 9 cells), indicating that it was unlikely to represent residual unblocked Ca²⁺-activated Cl^– current. This decaying component of current induced by the return to Na⁺-containing solution after Ca²⁺ loading therefore appears to represent electrogenic Na⁺-Ca²⁺ exchange in the olfactory receptor cilia.

This work was supported by the Wellcome Trust.