Diverse effects of pinacidil on KATP channels in mouse skeletal muscle in the presence of different nucleotides.

OBJECTIVE

The potassium channel opener pinacidil relaxes smooth muscle and exerts cardioprotective effects. The aim of the study was to investigate the actions of pinacidil on ATP sensitive potassium channels (KATP channels) in mammalian skeletal muscle and to explore the interrelations of this drug with various nucleotides.

METHODS

Single skeletal muscle fibres were prepared enzymatically from flexor digitorum brevis muscles of adult mice. Membrane patches of the inside-out configuration were excised in a Ca(2+)-free solution, and currents through single KATP channels were recorded at -40 mV. The cytoplasmic face of the patch was exposed to a K(+)-rich solution with MgCl2 (1 mM), and pinacidil (0.1 or 0.4 mM) and the nucleotides (0.1 mM) were added to this internal solution.

RESULTS

KATP channels were not activated by pinacidil in the presence of the nonhydrolysable ATP analogue AMP-PNP, in contrast to the reported channel activation by pinacidil and ATP. KATP channels had a high activity in the control and were blocked by ADP; the subsequent addition of pinacidil did not enhance the open probability of KATP channels. Pinacidil in the presence of a mixture of AMP-PNP and ADP activated KATP channels.

CONCLUSIONS

The diverse effects of pinacidil are interpreted with a model of the KATP channel containing a binding site for pinacidil and two sites for nucleotides, one activatory (A) site and one inhibitory (I) site. Occupation of the A site by ATP or ADP activates the channel, while occupation of the I site by ATP, AMP-PNP, ADP closes the channel. Pinacidil activates the channel and displaces blockers from the I site only if the A site is occupied.