A Novel K _ATP Current in Cultured Neonatal Rat Atrial Appendage Cardiomyocytes

Abstract

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 H(2)O or by the K(+) channel openers diazoxide and cromakalim with EC(50) approximately 1 and 10 nmol/L, respectively. The activated atrial K(ATP) current was highly sensitive to glyburide, with an IC(50) 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 micromol/L diazoxide, and was inhibited by glyburide, with an IC(50) 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.

Expression of ATP-Sensitive K+ Channel Subunits during Perinatal Maturation in the Mouse Heart

“…In the immature rat heart, K ATP channel density is also high, but the single-channel conductance and the mean open time are the same as in the adult (2). Reports describing a smaller unitary conductance in neonatal rat atrium (4) suggested that there may be regional differences. The immature rat heart K ATP channel has a lower open probability, and the channels are much more sensitive to block by intracellular ATP (2,5).…”

“…In addition to these biophysical changes, there are reports that immature K ATP channels may have a different pharmacological profile. The neonatal atrial K ATP channel shows a unique functional and pharmacological profile resembling the pancreatic ␤ cell channel for its unusually high affinity for glibenclamide and diazoxide (4,5). These characteristics suggest that neonatal K ATP channels may differ in their molecular composition relative to the mature heart.…”

Sulfonylurea Receptors Type 1 and 2A Randomly Assemble to Form Heteromeric KATPChannels of Mixed Subunit Composition

“…7 D). Based on our results it seems very likely that heteromeric K ATP channels are formed in native cells coexpressing SUR1 and SUR2; this may explain the displayed current phenotypes, such as intermediate metabolic sensitivities and pharmacological profi les observed in such cells (Baron et al, 1999;Liss et al, 1999;Poitry et al, 2003;Yunoki et al, 2003). The physiological role of these heteromeric channels during metabolic stresses, as well as their responses to drugs such as sulfonylureas and potassium channel openers, remain to be established.…”

“…Contradictory to this study, several reports suggested/indicated that SUR1 and SUR2 can coassemble into functional channels. First, single K ATP channels obtained from rat atrial myocytes displayed both SUR1 and SUR2 properties (Baron et al, 1999). Second, three populations of dopaminergic neurons, expressing Kir6.2 with SUR1, SUR1+SUR2B, and SUR2B, displayed K ATP currents with high, intermediate, and low sensitivities, respectively, to tolbutamide and metabolic inhibition (Liss et al, 1999).…”

“…Kir6.1 and Kir6.2, being 64% identical, display overlapping tissue expressions and form heteromeric complexes when coexpressed (Babenko et al, 2000;Cui et al, 2001;Pountney et al, 2001;van Bever et al, 2004). On the contrary, based on the study from Giblin et al, it is generally accepted that SUR1 cannot coassemble with SUR2 to form K ATP channels (Giblin et al, 2002;Babenko, 2005;Neagoe and Schwappach, 2005;Tricarico et al, 2006), even though SUR1 and SUR2 share ‫%56ف‬ identity and are coexpressed in many cell types (Baron et al, 1999;Liss et al, 1999;Shi et al, 2005;Tricarico et al, 2006). In this study, we examine whether SUR1 and SUR2 can coassemble to form functional K ATP channels.…”