Expression of K channels in Xenopus laevis oocytes injected with poly(A+) mRNA from the insulin-secreting beta-cell line, HIT T15

Channel activators regulate ATP-sensitive potassium channel (KIR6.1) expression in chick cardiomyocytes

ATP-sensitive potassium channels (K(ATP)) are widely expressed and yet little is known about the mechanisms regulating their expression. Here we report that expression of chick heart Kir6.1 is regulated by channel activators. Activation of K(ATP) with either ATP depletion or pinacidil, up-regulated Kir6.1 mRNA 1.8- to 2.4-fold in cultured ventricular myocytes as measured by competitive PCR. Pinacidil treatment also increased Kir6.1 protein as detected using an antibody to Kir6.1. Glibenclamide, a K(ATP) inhibitor, completely blocked the pinacidil-induced increase in Kir6.1 levels. It appears that Kir6.1 is up-regulated by an unknown signal transduction pathway initiated by K(ATP) opening.

Distinct modes of blockade in cardiac ATP-sensitive K+ channels suggest multiple targets for inhibitory drug molecules

Elementary K+ currents were recorded at 19 degrees C in inside-out patches from cultured neonatal rat cardiocytes to elucidate the block phenomenology in cardiac ATP-sensitive K+ channels when inhibitory drug molecules, such as the sulfonylurea glibenclamide, the phenylalkylamine verapamil or sulfonamide derivatives (HE 93 and sotalol), are interacting in an attempt to stress the hypothesis of multiple channel-associated drug targets. Similar to their adult relatives, neonatal cardiac K(ATP) channels are characterized by very individual open state kinetics, even in cytoplasmically well-controlled, cell-free conditions; at -7 mV, tau open(1) ranged from 0.7 to 4.9 msec in more than 200 patches and tau open(2) from 10 to 64 msec--an argument for a heterogeneous channel population. Nevertheless, a common response to drugs was observed. Glibenclamide and the other inhibitory molecules caused long-lasting interruptions of channel activity, after cytoplasmic application, as if drug occupancy trapped cardiac K(ATP) channels in a very stable, nonconducting configuration. The resultant NPo depression was strongest with glibenclamide (apparent IC50 13 nmol/liter) and much weaker with verapamil (apparent IC50 9 mumol/liter), HE 93 (apparent IC50 29 mumol/liter) and sotalol (apparent IC50 43 mumol/liter) and may have resulted from the occupancy of a single site with drug-specific affinity or of two sites, the high affinity glibenclamide target and a distinct nonglibenclamide, low affinity target. Changes in open state kinetics, particularly in the transition between the O1 state and the O2 state, are other manifestations of drug occupancy of the channel. Any inhibitory drug molecule reduced the likelihood of attaining the O2 state, consistent with a critical reduction of the forward rate constant governing the O1-O2 transition. But only HE 93 (10 mumol/liter) associated (with an apparent association rate constant of 2.3 x 10(6) mol-1 sec-1) to shorten significantly tau open(2) to 60.6 +/- 6% of the pre-drug value, not the expected result when the entrance in and the exit from the O2 state would be drug-unspecifically influenced. Sotalol found yet another and definitely distinctly located binding site to interfere with K+ permeation; both enantiomers associated with a rate close to 5 x 10(5) mol-1 sec-1 with the open pore thereby flicker-blocking cardiac K(ATP) channels. Clearly, these channels accommodate more than one drug-binding domain.

Ca(2+)-activated K+ channels from an insulin-secreting cell line incorporated into planar lipid bilayers

This study evaluates the use of the planar lipid bilayer as a functional assay of Ca(2+)-activated K+ channel activity for use in purification of the channel protein. Ca(2+)-activated K+ channels from the plasma membrane of an insulin-secreting hamster Beta-cell line (HIT T15) were incorporated into planar lipid bilayers. The single channel conductance was 233 picoSiemens (pS) in symmetrical 140 mmol/l KCl and the channel was strongly K(+)-selective (PCl/PK = 0.046; PNa/PK = 0.027). Channels incorporated into the bilayer with two orientations. In 65% of cases, the probability of the channel being open was increased by raising calcium on the cis side of the bilayer (to which the membrane vesicles were added) or by making the cis side potential more positive. At a membrane potential of + 20 mV, which is close to the peak of the Beta-cell action potential, channel activity was half-maximal at a Ca2+ concentration of about 15 mumol/l. Charybdotoxin greatly reduced the probability of the channel being open when added to the side opposite to that at which Ca2+ activated the channel. These results resemble those found for Ca(2+)-activated K+ channels in native Beta cell membranes and indicate that the channel properties are not significantly altered by incorporation in a planar lipid bilayer.

Characterization of maintained voltage-dependent K(+)-channels induced in Xenopus oocytes by rat brain mRNA

The voltage-dependent K+ currents encoded by rat brain mRNA were studied in Xenopus oocytes after the voltage-dependent Na+ currents and the Ca(2+)-activated Cl- currents were eliminated pharmacologically. This paper describes the maintained K+ currents (IK), defined primarily by resistance to inactivation for 1 s at a holding potential of -40 mV. IK activates at potentials more positive than -60 to -70 mV and consists of both low-threshold and high-threshold components. IK is partially blocked by both tetraethyl ammonium (TEA) and 4-aminopyridine (4-AP), which appear to be blocking the same component. Long depolarizing pulses result in incomplete inactivation of IK; the inactivating component is inhibited by TEA. Sucrose density gradient fractionation partially resolves the RNA encoding the several components of IK; most IK arises from size classes between 3.8 and 9.5 kb. The study gives further evidence for the existence of numerous distinct RNA populations that encode brain K+ channels different from previously reported cloned K+ channels that have been expressed in Xenopus oocytes.