SKA-31, a novel activator of SK(Ca) and IK(Ca) channels, increases coronary flow in male and female rat hearts

Abstract

Aims: Endothelial SK(Ca) and IK(Ca) channels play an important role in the regulation of vascular function and systemic blood pressure. Based on our previous findings that small molecule activators of SK(Ca) and IK(Ca) channels (i.e. NS309 and SKA-31) can inhibit myogenic tone in isolated resistance arteries, we hypothesized that this class of compounds may induce effective vasodilation in an intact vascular bed, such as the coronary circulation.

Methods and results: In a Langendorff-perfused, beating rat heart preparation, acute bolus administrations of SKA-31 (0.01-5 µg) dose-dependently increased total coronary flow (25-30%) in both male and female hearts; these responses were associated with modest, secondary increases in left ventricular (LV) systolic pressure and heart rate. SKA-31 evoked responses in coronary flow, LV pressure, and heart rate were qualitatively comparable to acute responses evoked by bradykinin (1 µg) and adenosine (10 µg). In the presence of apamin and TRAM-34, selective blockers of SK(Ca) and IK(Ca) channels, respectively, SKA-31 and bradykinin-induced responses were largely inhibited, whereas the adenosine-induced changes were blocked by ∼40%; TRAM-34 alone produced less inhibition. Sodium nitroprusside (SNP, 0.2 μg bolus dose) evoked changes in coronary flow, LV pressure, and heart rate were similar to those induced by SKA-31, but were unaffected by apamin + TRAM-34. The NOS inhibitor L-NNA reduced bradykinin- and adenosine-evoked changes, but did not affect responses to either SKA-31 or SNP.

Conclusion: Our study demonstrates that SKA-31 can rapidly and reversibly induce dilation of the coronary circulation in intact functioning hearts under basal flow and contractility conditions.

Figure 1

SKA-31 transiently increases coronary flow in male and female hearts. (A and B) Simultaneous recordings of stimulus-evoked changes in coronary flow and LV developed pressure in isolated hearts from male and female rats, respectively. Changes in coronary flow were observed following acute single dose administrations of SKA-31, bradykinin (BK), and adenosine (ADO) in the amounts denoted beneath the flow tracing. Flow and pressure recordings in (A) and (B) are representative of six male and seven female heart preparations subjected to the same experimental protocol.

Figure 2

Quantification of the drug-induced changes in coronary flow, LV systolic pressure, and heart rate following acute bolus administration of SKA-31 (0.01–5 µg), bradykinin (BK, 1 µg), adenosine (ADO, 10 µg), and solvent vehicle (Veh) in male and female hearts. Values are expressed as a percentage increase in a given parameter relative to the baseline value recorded immediately prior to the administration of a given compound. The histogram in (A) quantifies drug-induced changes in total coronary flow. (B and C) Evoked changes in LV systolic pressure and heart rate, respectively, in response to individual drug administrations. Data are presented as mean ± SEM, *P < 0.05 vs. female hearts.

Figure 3

Treatment with apamin and/or TRAM-34 inhibits the evoked responses to SKA-31, bradykinin, and adenosine, but not SNP, in male hearts. (A) Representative tracings of coronary flow (upper) and LV developed pressure (lower) in response to acute administrations of SKA-31 (0.5 and 5 µg), SNP (0.2 µg), bradykinin (BK, 1 µg), and adenosine (ADO, 10 µg) prior to and in the continued presence of apamin (0.1 µM) and TRAM-34 (1 µM). (B–D) Histograms quantifying SKA-31, BK, ADO, and SNP evoked increases in coronary flow (B), LV systolic pressure (C), and heart rate (D) under control conditions, and then in either the presence of apamin + TRAM-34 or TRAM-34 alone. Data are presented as mean ± SEM, *P < 0.05 vs. evoked response in the absence of apamin + TRAM-34.

Figure 4

Exposure to apamin and/or TRAM-34 inhibits evoked responses to SKA-31, bradykinin, and adenosine, but not SNP, in female hearts. (A) Representative tracings of coronary flow (upper) and LV developed pressure (lower) in response to acute administration of SKA-31 (0.5 and 5 µg), SNP (0.2 µg), bradykinin (BK, 1 µg), and adenosine (ADO, 10 µg) in the absence and in the continued presence of apamin (0.1 µM) and TRAM-34 (1 µM). (B–D) Histograms quantifying SKA-31, BK, ADO, and SNP evoked increases in coronary flow (B), LV systolic pressure (C), and heart rate (D) under control conditions, and then in either the presence of apamin + TRAM-34, or TRAM-34 alone. Data are presented as mean ± SEM, *P < 0.05 vs. evoked response in the absence of apamin + TRAM-34.

Figure 5

The NO synthase inhibitor L-NNA does not impact the SKA-31-evoked increase in coronary flow. (A) Representative tracings of coronary flow (upper) and LV developed pressure (lower) in a male rat heart in response to acute administration of SKA-31 (0.5 and 5 µg), SNP (0.2 µg), bradykinin (BK, 1 µg), and adenosine (ADO, 10 µg) in the absence and continued presence of 0.1 mM L-NNA. Histograms quantifying the SKA-31, BK, ADO, and SNP evoked changes in coronary flow, LV systolic pressure, and heart rate for male and female tissues are displayed in (B)–(D), respectively. Data are given as mean ± SEM, *P < 0.05 vs. evoked response in the absence of L-NNA.