In 1998, Edwards et al. proposed that potassium ions could be an endothelial-derived hyperpolarising factor (EDHF). We are now investigating the influence of extracellular potassium on phenylephrine (PE)-induced contractions of arterial smooth muscle isolated from Wistar rats, killed according to Schedule 1 of the Animals (Scientific Procedures) Act, 1986. (Data are given as means ± S.E.M. for n observations.)
Arteries with (+E) or without (-E) an endothelium were mounted for measurement of isometric force. A concentration-effect curve for PE was constructed, and thereafter arteries were contracted by PE to 70 % of the maximum response and relaxations measured against this contraction. Raising [K+]o from 5.9 to 11.2 mM did not relax either +E (96.3 ± 8.4 %, n = 9) or -E arteries (152 ± 13 %, n = 6) (see also Doughty et al. 2000). When [K+]o was first lowered to 1.18 mM, restoring [K+]o to 5.9 mM produced a relaxation (+E: 17.8 ± 4.2 %, n = 10; -E: 48.6 ± 10.5 %, n = 9). In -E arteries, the relaxation was unaffected by 30 µM Ba2+ (Doughty et al. 2000), but in both +E and -E relaxations were abolished by 1 mM ouabain (inhibitor of the Na+/K+-ATPase) (+E: 127.33 ± 12 %, n = 7; -E: 145.5 ± 19 %, n = 6). After lowering [K+]o to 1 mM for 3 min, restoration of [K+]o to between 10 and 40 mM produced a transient relaxation, whereas concentrations higher than 40 mM produced only contraction. Lowering [K+]o to 1.18 mM for periods longer than 30 s was sufficient to elicit relaxation when [K+]o was restored to 40 mM.
The relaxation evoked by reductions in extracellular potassium (low-K+) could be due to build-up of intracellular sodium and the subsequent overrun of electrogenic sodium-potassium ATPase (Na+/K+-ATPase) pumping when extracellular potassium was restored. Three things, however, do not fit easily with such a mechanism:
(1) The low-K+ salt solution contains 1.18 mM K+, which should support some Na+/K+-ATPase activity and the time of exposure was brief (3 min), limiting the build-up of intracellular sodium.
(2) The relaxation produced when potassium was restored was very long lasting, in excess of 1 h in some cases.
(3) Restoring potassium to concentrations that should depolarize significantly (~40 mM) continued to elicit relaxation, although the relaxations became increasingly transient at high concentrations.
These preliminary results suggest that the endothelium is not essential to observe low to high K+-induced relaxation, but that it involves a mechanism that is sensitive to block by oubain.
- Doughty, J.M., Boyle, J.P. & Langton, P.D. (2000). Br. J. Pharmacol. 130, 1174-1182.
Edwards, G., Dora, K.A., Gardener, M.J., Garland, C.J. & Weston, A.H. (1998). Nature 396, 269-272.