A comparison of responses to raised extracellular potassium and endothelium-derived hyperpolarizing factor (EDHF) in rat pressurised mesenteric arteries

The present study examined the hypothesis that potassium ions act as an endothelium-derived hyperpolarizing factor (EDHF) released in response to ACh in small mesenteric arteries displaying myogenic tone. Small mesenteric arteries isolated from rats were set up in a pressure myograph at either 60 or 90 mmHg. After developing myogenic tone, responses to raising extracellular potassium were compared to those obtained with ACh (in the presence of nitric oxide synthase and cyclo- oxygenase inhibitors). The effects of barium and ouabain, or capsaicin, on responses to raised extracellular potassium or ACh were also determined. The effects of raised extracellular potassium levels and ACh on membrane potential, were measured using sharp microelectrodes in pressurised arteries. Rat small mesenteric arteries developed myogenic tone when pressurised. On the background of vascular tone set by a physiological stimulus (i.e pressure), ACh fully dilated the small arteries in a concentration-dependent manner. This response was relatively insensitive to the combination of barium and ouabain, and insensitive to capsaicin. Raising extracellular potassium produced a more inconsistent and modest vasodilator response in pressurised small mesenteric arteries. Responses to raising extracellular potassium were sensitive to capsaicin, and the combination of barium and ouabain. ACh caused a substantial hyperpolarisation in pressurized arteries, while raising extracellular potassium did not. These data indicate that K+ is not the EDHF released in response to ACh in myogenically active rat mesenteric small arteries. Since the hyperpolarization produced by ACh was sensitive to carbenoxolone, gap junctions are the likely mediator of EDH responses under physiological conditions

ACh produces a larger vasodilator response than raising extracellular potassium in myogenically-active mesenteric small arteries.

<p>Vasodilator responses to (A) ACh and (B) raised extracellular potassium in rat isolated mesenteric small arteries pressurised to 60 mmHg or 90 mmHg (in the presence of L-NAME and indomethacin). Following the development of stable myogenic tone, cumulative concentration response curves were constructed to ACh or potassium in random order in the same artery from <i>n</i> different animals. Each point represents the mean ± s.e.mean (n = 6–9).</p

Vasodilator responses to raised extracellular potassium correlate with the degree of myogenic tone.

<p>The relationship between the level of myogenic tone (in the presence of L-NAME (100 µM) and indomethacin (1 µM)) and the R_max for ACh (<i>n</i> = 31) or raised extracellular potassium (<i>n</i> = 20) in rat isolated mesenteric arteries pressurised to 90 mmHg. Each point represents one experiment. The slope of the line fitted to the potassium data was significantly different from zero (<i>P</i><0.05) with an r² = 0.2387. The slope of the line fitted to the ACh data was not significantly different from zero.</p

ACh causes hyperpolarization in pressurized small arteries, while raising extracellular potassium does not.

<p>Summary data showing the change in membrane potential in smooth muscle cells from <i>n</i> different animals in response to ACh (1 µM) and raised extracellular potassium (+5 mM) at (A) 30 mmHg and (C) 90 mmHg. Following the successful impalement of a cell, responses to ACh or potassium were recorded in random order. Bars represent mean ± s.e.mean (<i>n</i> = 3–6). Representative trace recording (B) of the change in membrane potential of a rat mesenteric small artery held at 30 mmHg and exposed to ACh (1 µM) followed by raised extracellular potassium (+5 mM). The large deflections at the start and end of the trace recording reflect cell impalement and removal of the microelectrode, respectively.</p

Responses to raised extracellular potassium, but not ACh, are capsaicin sensitive.

<p>The effect of capsaicin on responses to raised extracellular potassium (A) or ACh (B) in rat isolated mesenteric small arteries at 90 mmHg. All experiments were carried out presence of L-NAME (100 µM and indomethacin (1 µM). Each point represents the mean ± s.e.mean. ***<i>P</i><0.001 capsaicin versus control; <i>n</i> = 4–5.</p