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

Abstract

Prevailing data suggest that sarcolemmal ATP-sensitive (K ATP ) channels in the adult heart consist of Kir6.2 and SUR2A subunits, but the expression of other K ATP channel subunits (including SUR1, SUR2B, and Kir6.1) is poorly defined. The situation is even less clear for the immature heart, which shows a remarkable resistance to hypoxia and metabolic stress. The hypoxia-induced action potential shortening and opening of sarcolemmal K ATP channels that occurs in adults is less prominent in the immature heart. This might be due in part to the different biophysical and pharmacological properties of K ATP channels of immature and adult K ATP channels. Because these properties are largely conferred by subunit composition, it is important to examine the relative expression levels of the various K ATP channel subunits during maturation. We therefore used RNAse protection assays, reverse transcription-PCR approaches, and Western blotting to characterize the mRNA and protein expression profiles of K ATP channel subunits in fetal, neonatal, and adult mouse heart. Our data indicate that each of the K ATP channel subunits (Kir6.1, Kir6.2, SUR1, SUR2A, and SUR2B) is expressed in the mouse heart at all of the developmental time points studied. However, the expression level of each of the subunits is low in the fetal heart and progressively increases with maturation. Each of the subunits seems to be expressed in ventricular myocytes with a subcellular expression pattern matching that found in the adult. Our data suggest that the K ATP channel composition may change during maturation, which has important implications for K ATP channel function in the developing heart. ATP-sensitive K ϩ (K ATP ) channels are robustly expressed in the immature heart (1,2). However, K ATP channels in the immature heart may differ in several respects to those of adult cardiac tissue. In immature rabbit ventricle, K ATP channel open probability is roughly the same as in the adult, but they exhibit a significantly smaller single-channel conductance (~55 pS) and channel density, as defined by the number of open channels per patch (3). 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. K ATP channels are believed to be hetero-...

Sulfonylurea Receptor-Dependent and -Independent Pathways Mediate Vasodilation Induced by ATP-Sensitive K+ Channel Openers

“…As a positive control, SUR subtypes expressed in SUR(ϩ/ϩ) whole heart were examined. Transcripts for SUR1, SUR2A, and SUR2B were amplified from whole heart cDNA, consistent with previous studies (Isomoto et al, 1996;Morrissey et al, 2005), whereas only SUR2B was detected in mouse arterial smooth muscle cells. These data indicate that SUR2B is the only SUR subtype that is expressed in mesenteric artery smooth muscle cells.…”

Large-scale analysis of ion channel gene expression in the mouse heart during perinatal development

“…We found expression of each of these subunits in adult heart and clear upregulation during development. This result is in keeping with earlier observations of increased K ATP channel expression with development (56) and developmental upregulation of these K ATP channel subunits (37).…”

Consequences of cardiac myocyte-specific ablation of KATPchannels in transgenic mice expressing dominant negative Kir6 subunits

“…Western blotting was performed using crude membrane fractions obtained from mouse heart and brain using previously described procedures (18). Equal amount of proteins were separated on 12% SDS-PAGE, transferred to polyvinylidene difluoride membranes, and immunoblotted with anti-Kir6.1 NAF-1 (1: 1,500) or anti-Kir6.2 G16 antibody (1: 500, Santa Cruz Biotechnology).…”