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Background and Purpose: The canonical Kir6.2/SUR2A ventricular KATP channel is highly ATP‐sensitive and remains closed under normal physiological conditions. These channels activate only when prolonged metabolic compromise causes significant ATP depletion and then shortens the action potential to reduce contractile activity. Pharmacological activation of KATP channels is cardioprotective, but physiologically, it is difficult to understand how these channels protect the heart if they only open under extreme metabolic stress. The presence of a second KATP channel population could help explain this. Here, we characterise the biophysical and pharmacological behaviours of a constitutively active Kir6.1‐containing KATP channel in ventricular cardiomyocytes. Experimental Approach: Patch‐clamp recordings from rat ventricular myocytes in combination with well‐defined pharmacological modulators was used to characterise these newly identified K+ channels. Action potential recording, calcium (Fluo‐4) fluorescence measurements and video edge detection of contractile function were used to assess functional consequences of channel modulation. Key Results: Our data show a ventricular K+ conductance whose biophysical characteristics and response to pharmacological modulation were consistent with Kir6.1‐containing channels. These Kir6.1‐containing channels lack the ATP‐sensitivity of the canonical channels and are constitutively active. Conclusion and Implications: We conclude there are two functionally distinct populations of ventricular KATP channels: constitutively active Kir6.1‐containing channels that play an important role in fine‐tuning the action potential and Kir6.2/SUR2A channels that activate with prolonged ischaemia to impart late‐stage protection against catastrophic ATP depletion. Further research is required to determine whether Kir6.1 is an overlooked target in Comprehensive in vitro Proarrhythmia Assay (CiPA) cardiac safety screens. [ABSTRACT FROM AUTHOR]

1 We investigated the mechanism of action of two novel nonsulphonylurea ATP-sensitive potassium channel (K[SUBATP]) inhibitors, PNU-99963 and PNU-37883A, on four types of cloned K[SUBATP] channels. 2 Whole-cell currents were recorded in a symmetrical potassium (140 mM) gradient in HEK-293 cells stably expressing Kir6.2/SUR1, Kr6.2/SUR2A. Kr6.2/SUR2B or Kir6.1/SUP2B. 3 PNU-99963 potently inhibited the four K[SUBATP] channel clones. The concentration at which half-maximum current was inhibited (IC[SUB50]) was 66, 41, 43 and 11 nM for Kir6.2/SUR1, KR6.2/SUR2A, Kir6.2/SUR2B and Kir6.1/SUR2B, respectively. In contrast, PNU-99963 up to a concentration of 3μM had no significant effect on current generated in HEK-293 cells by transiently expressing Kir6.2Δ26, a C-terminal truncated pore-forming subunit of Kir6.2 4 PNU-37883A inhibited four types of K[SUBATP] channels, but to different extents. Inhibition of the putative smooth muscle K[SUBATP] channel types. Kir6.2/SUR2B (IC[SUB50]; 15 μM) and Kir6.1/SUR2B (IC[SUB50]; 6 μM), was significantly greater than inhibition of either the pancreatic β cell or cardiac K[SUBATP] channel clones. Moreover, PNU-37883A significantly inhibited currents generated by expressing Kir6.2Δ26 alone, with an IC[SUB50] of 5 μM, which was significantly increased to 38 μM when Kir6.2Δ26 was expressed with SUR2B. 5 In conclusion, two structurally different nonsulphonylurea compounds, PNU-99963 and PNU-37883A, inhibit K[SUBATP] channels via different mechanisms, namely through the sulphonylurea receptor (SUR), and the pore-forming subunits, respectively, although SUR2B reduced the inhibitory effects of PNU-37883A. While PNU-99963 potently inhibits all the four cloned K[SUBATP] channels, PNU-37883A has a degree of selectively towards both smooth muscle K[SUBATP] channels, but could not discriminate between them. [ABSTRACT FROM AUTHOR]

Background and Purpose: The canonical K ir 6.2/SUR2A ventricular K ATP channel is highly ATP-sensitive and remains closed under normal physiological conditions. These channels activate only when prolonged metabolic compromise causes significant ATP depletion and then shortens the action potential to reduce contractile activity. Pharmacological activation of K ATP channels is cardioprotective, but physiologically, it is difficult to understand how these channels protect the heart if they only open under extreme metabolic stress. The presence of a second K ATP channel population could help explain this. Here, we characterise the biophysical and pharmacological behaviours of a constitutively active K ir 6.1-containing K ATP channel in ventricular cardiomyocytes., Experimental Approach: Patch-clamp recordings from rat ventricular myocytes in combination with well-defined pharmacological modulators was used to characterise these newly identified K + channels. Action potential recording, calcium (Fluo-4) fluorescence measurements and video edge detection of contractile function were used to assess functional consequences of channel modulation., Key Results: Our data show a ventricular K + conductance whose biophysical characteristics and response to pharmacological modulation were consistent with K ir 6.1-containing channels. These K ir 6.1-containing channels lack the ATP-sensitivity of the canonical channels and are constitutively active., Conclusion and Implications: We conclude there are two functionally distinct populations of ventricular K ATP channels: constitutively active K ir 6.1-containing channels that play an important role in fine-tuning the action potential and K ir 6.2/SUR2A channels that activate with prolonged ischaemia to impart late-stage protection against catastrophic ATP depletion. Further research is required to determine whether K ir 6.1 is an overlooked target in Comprehensive in vitro Proarrhythmia Assay (CiPA) cardiac safety screens., (© 2024 The Authors. British Journal of Pharmacology published by John Wiley & Sons Ltd on behalf of British Pharmacological Society.)