Vasodilation of rat skeletal muscle arteries by the novel BK channel opener GoSlo is mediated by the simultaneous activation of BK and Kv 7 channels

Background and Purpose

BK channels play important roles in various physiological and pathophysiological processes and thus have been the target of several drug development programmes focused on creating new efficacious BK channel openers, such as the GoSlo‐SR compounds. However, the effect of GoSlo‐SR compounds on vascular smooth muscle has not been studied. Therefore, we tested the hypothesis that GoSlo‐SR compounds dilate arteries exclusively by activating BK channels.

Experimental Approach

Experiments were performed on rat Gracilis muscle, saphenous, mesenteric and tail arteries using isobaric and isometric myography, sharp microelectrodes, digital droplet PCR and the patch‐clamp technique.

Key Results

GoSlo‐SR compounds dilated isobaric and relaxed and hyperpolarised isometric vessel preparations and their effects were abolished after (a) functionally eliminating K⁺ channels by pre‐constriction with 50 mM KCl or (b) blocking all K⁺ channels known to be expressed in vascular smooth muscle. However, these effects were not blocked when BK channels were inhibited. Surprisingly, the K_v7 channel inhibitor XE991 reduced their effects considerably, but neither K_v1 nor K_v2 channel blockers altered the inhibitory effects of GoSlo‐SR. However, the combined blockade of BK and K_v7 channels abolished the GoSlo‐SR‐induced relaxation. GoSlo‐SR compounds also activated K_v7.4 and K_v7.5 channels expressed in HEK 293 cells.

Conclusion and Implications

This study shows that GoSlo‐SR compounds are effective relaxants in vascular smooth muscle and mediate their effects by a combined activation of BK and K_v7.4/K_v7.5 channels. Activation of K_v1, K_v2 or K_v7.1 channels or other vasodilator pathways seems not to be involved.

Rho-associated kinase plays a role in rabbit urethral smooth muscle contraction, but not via enhanced myosin light chain phosphorylation

The involvement of Rho-associated kinase (ROK) in activation of rabbit urethral smooth muscle contraction was investigated by examining the effects of two structurally distinct inhibitors of ROK, Y27632 and H1152, on the contractile response to electric field stimulation, membrane depolarization with KCl, and α1-adrenoceptor stimulation with phenylephrine. Both compounds inhibited contractions elicited by all three stimuli. The protein kinase C inhibitor GF109203X, on the other hand, had no effect. Urethral smooth muscle strips were analyzed for phosphorylation of three potential direct or indirect substrates of ROK: 1) myosin regulatory light chains (LC20) at S19, 2) the myosin-targeting subunit of myosin light chain phosphatase (MYPT1) at T697 and T855, and 3) cofilin at S3. The following results were obtained: 1) under resting tension, LC20 was phosphorylated to 0.65±0.02 mol Pi/mol LC20 (n=21) at S19; 2) LC20 phosphorylation did not change in response to KCl or phenylephrine; 3) ROK inhibition had no effect on LC20 phosphorylation in the absence or presence of contractile stimuli; 4) under resting conditions, MYPT1 was partially phosphorylated at T697 and T855 and cofilin at S3; 5) phosphorylation of MYPT1 and cofilin was unaffected by KCl or phenylephrine; and 6) KCl- and phenylephrine-induced contraction-relaxation cycles did not correlate with actin polymerization-depolymerization. We conclude that ROK plays an important role in urethral smooth muscle contraction, but not via inhibition of MLCP or polymerization of actin.

Origin of spontaneous rhythmicity in smooth muscle

Rhythmic electrical activity is a feature of most smooth muscles but the mechanical consequences can vary from regular rapid phasic contractions to sustained contracture. For many years it was thought that spontaneous electrical activity originated in smooth muscle cells but recently it has become apparent that there are specialized pacemaker cells in many organs that are morphologically and functionally distinct from smooth muscle and that the former cells are the source of spontaneous electrical activity. Such a pacemaker function is well documented for the ICC of the gastrointestinal tract but evidence is accumulating that ICC-like cells play a similar role in other types of smooth muscle. We have recently shown that there are specialized pacemaking cells in the rabbit urethra which are spontaneously active when freshly isolated, readily distinguishable from smooth muscle cells under bright field illumination and relatively easy to study using patch-clamp and confocal imaging techniques. Recent results suggest that calcium oscillations in isolated rabbit urethral interstitial cells are initiated by calcium release from ryanodine sensitive intracellular stores, that oscillation frequency is very sensitive to the external calcium concentration and that conversion of the primary oscillation to a propagated calcium wave depends upon IP3-induced calcium release.

Peripheral lymphatic responses to outflow pressure in anaesthetized sheep

1. Lymph flow and pressure fluctuations were measured by cannulating popliteal efferent and distal hind-limb afferent lymphatic vessels in anaesthetized sheep. The cannula outflow height was raised above the vessels to increase lymphatic outflow pressure. 2. Lymph flow decreased non-linearly as the outflow was raised. The rate of decrease increased with increasing outflow height. 3. Lymphatic contraction frequency rose and stroke volume fell with increasing outflow height. 4. The calculated power necessary to move lymph along the cannula initially increased with outflow height but it reached a peak and was reduced again by raising the outflow further. Calculated lymphatic stroke work followed a very similar pattern. 5. Lymph flow was maintained up to a greater outflow height in afferent than in efferent vessels. Curves relating frequency, power and stroke work to outflow height were shifted to the right in the afferent lymphatics. 6. These results are consistent with an intrinsic lymphatic pump which can be stimulated by increasing pressure. At high pressures, however, the pump fails.

A method for studying lymphatic pumping activity in conscious and anaesthetized sheep

A 20 cm length of the main mesenteric lymph duct in the sheep was cannulated at both ends and measurements were made in both conscious and anaesthetized animals of the ability of the duct to pump saline from an inflow reservoir to an outflow at the same height. Fluid was propelled only when the lymphatic contracted and this was unaffected by movements of the animal round the cage or by fluctuations in abdominal pressure or by respiratory movements. It was confirmed that the mesenteric duct was 'isolated' from the rest of the lymphatic system by closing the inflow tap whereupon fluid propulsion ceased. Raising transmural pressure by varying inflow and outflow by the same amount had the effect of increasing fluid output; this was achieved by an increase in both the frequency and force of lymphatic contractions. Lymphatic frequency of contraction and fluid output increased when the animals were frightened. Intravenous infusions of noradrenaline increased the frequency of lymphatic contraction and increased fluid propulsion, while isoprenaline infusions depressed flow. This preparation demonstrates that it is possible to study the control of lymphatic pumping in conscious sheep without the complication of changing rates of lymph formation.