A novel KATP current in cultured neonatal rat atrial appendage cardiomyocytes

The functional and pharmacological properties of ATP-sensitive K+ (K-ATP) channels were studied in primary cultured neonatal rat atrial appendage cardiomyocytes. Activation of a whole-cell inward rectifying K+ current depended on the pipette ATP concentration and correlated with a membrane hyperpolarization close to the K+ equilibrium potential. The K-ATP current could be activated either spontaneously or by a hypotonic stretch of the membrane induced by lowering the osmolality of the bathing solution from 290 to 260 mOsm/kg H2O or by the K+ channel openers diazoxide and cromakalim with EC50 approximate to 1 and 10 nmol/L, respectively. The activated atrial K-ATP current was highly sensitive to glyburide, with an IC50 of 1.22+/-0.15 nmol/L. Recorded in inside-out patches, the neonatal atrial K-ATP channel displayed a conductance of 58.0+/-2.2 pS and opened in bursts of 133.8+/-20.4 ms duration, with an open time duration of 1.40+/-0.10 ms and a close time duration of 0.66+/-0.04 ms for negative potentials. The channel had a half-maximal open probability at 0.1 mmol/L ATP, was activated by 100 mu mol/L diazoxide, and was inhibited by glyburide, with an IC50 in the nanomolar range. Thus, pending further tests at low concentrations of K-ATP channel openers, the single-channel data confirm the results obtained with whole-cell recordings. The neonatal atrial appendage K-ATP channel thus shows a unique functional and pharmacological profile resembling the pancreatic beta-cell channel for its high affinity for glyburide and diazoxide and for its conductance, but also resembling the ventricular channel subtype for its high affinity for cromakalim, its burst duration, and its sensitivity to ATP. Reverse transcriptase-polymerase chain reaction experiments showed the expression of Kir6.1, Kir6.2, SUR1A, SUR1B, SUR2A, and SUR2B subunits, a finding supporting the hypothesis that the neonatal atrial K-ATP channel corresponds to a novel heteromultimeric association of K-ATP channel subunits.