Action of cromakalim on potassium membrane conductance in isolated heart myocytes of frog

SR47063, a potent channel opener, activates KATP and a time-dependent current likely due to potassium accumulation

(i) We studied the effects of a new cromakalim analogue, SR47063, in guinea-pig ventricular cells. The experiments were carried out in whole-cell patch clamp with internal and external solutions supposedly similar to the physiological ones. (ii) SR47063 reversibly activated a time-independent current reversing near the potassium equilibrium potential, and a time-dependent current reversing at a more positive potential. Both currents were blocked by application of glibenclamide. (iii) The time-independent and the time-dependent currents were activating for the same concentration of agonist in every cell, this concentration being very different from cell to cell. (iv) The amplitude of the time-dependent current was shown to depend directly neither on agonist concentration nor on potential, but rather on the amplitude of the current flowing during the prepulse before the test pulse. (v) We conclude that SR47063 is a potent KATP channel opener acting at concentrations lower than one micromolar, and that the time-dependent current is likely due to accumulation and depletion of potassium in restricted areas of the cells.

Effects of ATP-sensitive K(+)-channel activators on transmitter release parameters at the frog neuromuscular junction

The frog neuromuscular junction was used to study KATP channel action and its relation to transmitter release. Diazoxide (10 microM) and cromakalim (300 microM) decreased the amplitude of the end-plate potential (EPP) without significantly affecting the miniature end plate potential (MEPP) amplitude. Thus, there was a significant decrease in the quantal content of EPP after administration of the KATP channel activators. These agents did not alter MEPP frequency or resting membrane potential (RMP). The diazoxide-induced decrease in EPP amplitude was antagonized by the KATP blocker glibenclamide (10 microM) suggesting KATP channel involvement. Glibenclamide (10 microM) by itself caused a significant decrease in the RMP without significantly affecting the parameters of transmitter release. The diazoxide-induced decrease in EPP amplitude was not readily reversible with washing, however, when glibenclamide was added after diazoxide, the effect was easier to reverse. The results indicate that the activators exert their effect predominantly at the presynaptic region.

Effects of potassium channel openers and calcium channel blockers on the force responses of the electrically driven rat right ventricle strip

1. The effects of potassium channel openers (cromakalim and pinacidil) and of calcium channel blockers (verapamil, diltiazem and flunarizine) on the contractile responses of the rat right ventricle have been determined. 2. On the electrically driven rat right ventricle cromakalim at 3 x 10(-7), 10(-6), 3 x 10(-6) M and 10(-5) M, and pinacidil at 10(-6), 10(-5) and 3 x 10(-5) M had no effect on the force responses to cardiac stimulation or to isoprenaline. 3. Pinacidil at 10(-4) M in a vehicle of 0.7% ethanol reduced the force responses to cardiac stimulation. This inhibitory effect was solely due to the ethanol. Pinacidil in 5 x 10(-4) M HCl (which had no effect alone) potentiated some of the responses to isoprenaline. 4. Verapamil, greater than or equal to 10(-6) M, diltiazem, greater than or equal to 10(-5) M, and flunarizine, greater than or equal to 10(-6) M, reduced the force responses to cardiac stimulation and to isoprenaline. 5. The present study has shown that calcium channel blockers, but not potassium channel openers, have inhibitory effects on the rat right ventricle.