Mouse renal inner medullary collecting duct cells (mIMCD-K2 cell-line) possess a Ca²⁺-activated Cl^– conductance (CaCC) which participates in transepithelial Cl^– secretion (Boese et al. 2000). We have now used whole cell patch clamp recordings to determine the dependence of CaCC upon intracellular [Ca²⁺]_i and noise analysis to estimate the conductance of the channel.

At 0.01 µM [Ca²⁺]_i only leakage currents were detected; at 0.1 µM [Ca²⁺]_i cell currents were very small. Only a slow outward relaxation was observed in response to the largest positive voltage (+80 mV) jump in some experiments whereas in others only leakage currents were seen. At 0.5 µM [Ca²⁺]_i relaxations of large amplitude were obtained. At 1.0 µM [Ca²⁺]_i and above, currents were large, but current relaxations were of small amplitude or not present, indicating that the channels were approaching full activation at all potentials tested. Fitting the [Ca²⁺]_i dependence of CaCC with a sigmoidal Hill equation gave EC₅₀ values of 650.5 ± 31.4 nM (mean ± S.E.M. at -80 mV) and 306.1 ± 44.6 nM (at +80 mV) with Hill coefficients of 3.0 ± 0.3 and 1.7 ± 0.4, respectively. Diversity in Ca²⁺-activated Cl^– channels is suggested by the widely different single-channel conductances (Kidd & Thorn, 2000) which range from 2 to 30 pS. In order to estimate the single channel conductance of CaCC in mIMCD-K2 cells both stationary and non-stationary noise analysis on whole cell currents were performed. Stationary noise analysis yielded a single channel conductance of 6.2 ± 0.8 pS and a channel density per cell of 5561 ± 311 (n = 5). Non-stationary noise analysis generated a similar single channel conductance of 7.1 ± 0.9 pS and a density of 4251 ± 251 (n = 6, n.s. versus stationary, unpaired Student’s t test).

In conclusion, our data indicate that CaCC in mIMCD-K2 cells is a small conductance Cl^– channel whose activity is tightly coupled to changes in [Ca²⁺]_i over the normal physiological range.

This work was supported by the NKRF