Phasic Oscillations of Extracellular Potassium (Ko) in Pregnant Rat Myometrium

K-sensitive microelectrodes were used to measure K + within the extracellular space (K o ) of pregnant rat myometrium. Contractile activity was monitored by measuring either force or bioelectrical signals. Single and double-barreled electrodes were used. Double-barreled electrodes allowed monitoring of electrical activity 15 microns from the site of K o measurement. From double-barreled electrode experiments, the bioelectrical burst started first, and then K o began to rise 0.6 ± 0.1 seconds later. This delay indicates that K + leaves the cells in response to local electrical activity rather than vice versa. Four control experiments were performed to assess the influence of electrical artifacts caused by tissue motion on K o values. When observed, artifacts were negative and transient, and hence would result in an underestimation of K o rises. Artifacts were minimized when tissue motion was minimized by fixing the tissue at both ends. At 37°C, 7 single barreled experiments and 45 contractions were analyzed. Resting K o was within 1 mM of bath K + (5 mM) at the beginning and end of the experiments. K o rose during the contraction, fell after the completion of the contraction, and normalized before the next contraction began. Peak K o values observed during force production were 18.8 ± 5.9 mM, a value high enough to modulate tissue-level electrical activity. K o required 15.7 ± 2.8 seconds to normalize halfway (t 50 ). Six experiments expressing 38 contractions were performed at 24°C. The contraction period was longer at 24°C. Values for peak K o (26.2 ± 9.9 mM) and t 50 (29.8±16.2 sec) were both larger than at 37°C (p<0.0003 for both). The direct relationships between peak K o , t 50 and the contraction period, suggest elevations in K o may modulate contraction frequency. The myometrial interstitial space appears to be functionally important, and K o metabolism may participate in cell-cell interactions.