DIFFERENT SULFONYLUREA AND ATP SENSITIVITY CHARACTERIZES THE JUVENILE FORM AND THE ADULT FORM OF KATP CHANNEL COMPLEX OF RAT SKELETAL MUSCLE FIBERS.

We have described here the changes of the biophysical and pharmacological properties of the sarcolemmal ATP-sensitive K+ channels (KATP) of rat skeletal muscle fibres, occurring from an early postnatal period (5 days) to adulthood (210 days). The age-dependent changes of the mean current of the KATP channel (channel activity) and the effects of the blockers, ATP and glybenclamide, were examined by using the patch-clamp technique. Measurements of the single channel conductance, open probability and channel density were also performed. Excision of cell-attached patches into an ATP-free solution dramatically increased the KATP channel activity; however, the intensity of this activity was age dependent. The relative activity was low at 5-6 days of postnatal life, increased to a plateau at 12-13 days, then declined toward adult values after 37 days. Two distinct types of the KATP channel complex could be distinguished. The early developmental period (5-6 days) was dominated by a KATP channel having a conductance of 66 pS, a high open probability of 0.602, and an IC50 for ATP and glybenclamide of 123.1 microM and 3.97 microM, respectively. This type of channel disappeared with maturation of the muscle to be replaced by the adult form of the KATP channel. The later developmental period (from 56 days) was dominated by a KATP channel having a 71 pS conductance, but a low open probability of 0.222. This adult channel was also 3.2 and 73.5 times more sensitive to ATP and glybenclamide, respectively. We have also observed that the sensitivity of the KATP channel to ATP and glybenclamide develops differently. Indeed, the greater increase in the sensitivity of the channel to ATP was observed between 5 and 12 days of age. Conversely, the greater enhancement of the sensitivity of the channel to glybenclamide occurred between 12 and 37 days. A further increase of this parameter was also observed between 37 and 56 days of age. The differential age-dependent acquisition of the sensitivity of KATP channels to ATP and glybenclamide poses the hypothesis that in rat skeletal muscle the ATP regulatory site and sulfonylurea site are located on different subunits of the KATP channel complex. The intense KATP channel activity recorded between 12 and 37 days of postnatal life sustains the high resting macroscopic K+ conductance characteristic of the early postnatal development.