Mice lacking sulfonylurea receptor 2 (SUR2) ATP-sensitive potassium channels are resistant to acute cardiovascular stress

Abstract

Adenosine triphosphate-sensitive potassium (K(ATP)) channels are thought to mediate the stress response by sensing intracellular ATP concentration. Cardiomyocyte K(ATP) channels are composed of the pore-forming Kir6.2 subunit and the regulatory sulfonylurea receptor 2 (SUR2). We studied the response to acute isoproterenol in SUR2 null mice as a model of acute adrenergic stress and found that the episodic coronary vasospasm observed at baseline in SUR2 null mice was alleviated. Similar results were observed following administration of a nitric oxide donor consistent with a vasodilatory role. Langendorff-perfused hearts were subjected to global ischemia, and hearts from SUR2 null mice exhibited significantly reduced infarct size (54+/-4 versus 30+/-3%) and improved cardiac function compared to control mice. SUR2 null mice have hypertension and develop cardiac hypertrophy. However, despite longstanding hypertension, fibrosis was absent in SUR2 null mice. SUR2 null mice were administered nifedipine to block baseline coronary vasospasm, and hearts from nifedipine-treated SUR2 null mice exhibited increased infarct size compared to untreated SUR2 null mice (42+/-3% versus 54+/-3%). We conclude that conventional sarcolemmal cardiomyocyte K(ATP) channels containing full-length SUR2 are not required for mediating the response to acute cardiovascular stress.

Figure 1

Heart rate response to isoproterenol injection. A. Isoproterenol injection at 5 μg/gm body mass was used to induce acute β adrenergic stress in SUR2 null and strain matched control mice. The peak heart rate after injection was similar between mutant and controls. DETA/NO injections similarly produced an increase in heart rate. B. Average duration of tachycardia in control and SUR2 null mice following administration of isoproterenol and nitric oxide donor (DETA/NO, 120 μg/gm body mass). (*p<0.05 vs baseline)

Figure 2

Isoproterenol decreased ST segment elevation in SUR2 mice. A. Baseline ECG recordings of control and SUR2 null mice. SUR2 null mice exhibit spontaneous episodes of coronary vasospasm evidenced by ST segment elevation on ECG. Episodes vary in length and usually resolve within 3 minutes; an example of vasospasm is shown under SUR2 KO. B. ECG recordings immediately following isoproterenol injection. Control mice (n=3/3) exhibited ST segment depression, while no ST segment depression or other ECG abnormality was observed in SUR2 null mice during the resulting tachycardic period (n=0/5). C. Quantification of ST segment elevation before and after drug administration in SUR2 null mice. While coronary vasospasm was present in all SUR2 null mice at baseline, none was observed during tachycardia.

Figure 3

Effect of global ischemia on SUR2 null hearts. A. Isolated hearts from control and SUR2 null mice were exposed to 40 minutes of “no-flow” ischemia via a standard Langendorff setup. Following reperfusion, hearts were sectioned and stained with 1% TTC to visualize infarct size. Representative heart sections from control and SUR2 null mice are shown. Viable tissue stains dark red (black) while infracted tissue becomes white and drained of color. B. Infarct size was quantified for control and SUR2 null hearts. SUR2 null mice exhibit significantly smaller infarcts following 40 minutes of global no flow ischemia. * p<0.001, n=10 and n=8 for control and SUR2 null, respectively.

Figure 4

Effect of ischemia on cardiac function in SUR2 null hearts. Left ventricular pressures were measured using a pressure-sensing catheter connected to a fluid filled balloon placed inside the left ventricle. A. Representative examples of left ventricular pressure at baseline and after 15 minutes of reperfusion are shown. Hearts from SUR2 null mice exhibit improved conserved left ventricular function following ischemia versus control hearts. B. Average systolic and diastolic pressures for control and SUR2 null mice. SUR2 null hearts exhibited significantly greater systolic pressures throughout reperfusion. Diastolic pressures were similar between cohorts, though a trend of higher diastolic pressures in control hearts immediately following ischemia was observed. * p<0.001, n=10 and n=7 for control and SUR2 null, respectively. C. Left ventricular developed pressures were significantly greater in SUR2 null hearts during reperfusion compared to controls even though baseline LVDP was similar between cohorts. ** p<0.01, * p<0.05, # p<0.0001 for two-way ANOVA comparison of cohorts, n=10 and n=7 for control and SUR2 null, respectively.

Figure 5

Cardiac hypertrophy in SUR2 null mice. A. Representative photomicrographs of hemotoxylin and eosin and Masson trichrome stained sections from control and SUR2 null hearts. Despite significant increases in heart size, SUR2 null hearts exhibited no fibrosis, cardiomyocyte disarray or other structural defects. (Scale bar = 0.5 mm) B. Hearts from SUR2 null mice are significantly larger than matched control hearts as measured by ventricular weight or relative heart size (ventricular weight/body weight, gm/gm). * p<0.05, n=16 and n=20 for control and SUR2 null cohorts, respectively. C. Average body weight of the control and SUR2 null cohorts were not significantly different.

Figure 6

Reducing coronary vasospasm partially blocks cardioprotection in SUR2 mice. Nifedipine (3 mg/kg body weight) was chronically administered via intraperitoneal mini-osmotic pumps for at least 7 days. Ambulatory ECG monitoring was conducted on a subset of the cohort (n=3/8) to ensure reduction of vasospasm. A. Nifedipine significantly reduced vasospasm in SUR2 null mice. Total duration of ST segment elevation 12 hours after pump implantation (“baseline”) and after 7 days (“NIF”) was quantified. Nifedipine significantly reduced total duration of vasospasm, as well as the incidence and length of vasospasm episodes. * p<0.05, n=3. B. Hearts isolated from nifedipine treated SUR2 null mice underwent Langendorff ischemia experiments. Average infarct size was significantly greater in nifedipine-treated SUR2 null hearts compared SUR2 null animals, though infarct size was still less that control. * p<0.05, n=8 per group. C. Left ventricular developed pressure was not significantly different between nifedipine treated and untreated SUR2 null cohorts (n=8 and n=7, respectively).