• The Korean Journal of Physiology
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• v.27 no.2
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• pp.199-207
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• 1993

To study the underlying mechanism through which the endothelium-dependent relaxation is inhibited by blocking the $Na^+\;-K^+$ pump, the effects of $Na^+\;-K^+$ pump blockade on the release of EDRF and its relaxing activity were examined, using organ bath study, bioassay technique, and cGMP measurement. Endothelium-dependent relaxation was attenuated by blocking the $Na^+\;-K^+$ pump in the vascular ring with intact endothelium. In bioassay experiment EDRF release was decreased with the blockade of the $Na^+\;-K^+$ pump in the EDRF donor strip. Endothelium-dependent increase of cGMP level was suppressed by inhibiting the $Na^+\;-K^+$ pump in the test strips. The magnitude of relaxation of test strip which was induced by the perfusate that had passed through the EDRF donor strip was decreased with the blockade of the $Na^+\;-K^+$ pump in the test strip. Therefore, it could be suggested that the attenuation of endothelium-dependent relaxation caused by inhibiting $Na^+\;-K^+$ pump activity is due to both the decreased release of EDRF from endothelial cells and the decreased sensitivity of the smooth muscle cells to EDRF.

• The Korean Journal of Physiology and Pharmacology
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• v.2 no.1
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• pp.63-68
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• 1998

Nitric oxide (NO)-mediated relaxation in vascular smooth muscle involves not only activation of guanylate cyclase but also hyperpolarization of the membrane. It has been shown that depolarization decreases the [$Ca^{2+}$] sensitivity of myosin light chain kinase in arterial smooth muscle, and nitric oxide (NO)-mediated relaxation was attenuated in this situation. However, why potassium inhibits or attenuates the action of EDRF/NO is not clear. Therefore, we investigated the magnitude of relaxation and cGMP contents using measures known to release NO, such as photorelaxation, photo activated NO-mediated relaxation, and NO-donor (SNP)-mediated relaxation in porcine coronary arterial rings in which contractile conditions were made by different degree of depolarization, i.e., contraction in response to U46619 or U46619 plus KCl. In all cases, the magnitude of relaxation was significantly greater (P＜0.05) in U46619-contracted rings than in U46619+KCl-contracted ones. Although accumulation of cGMP was evident with three measures employed in the present study, no difference was found in cGMP contents between U46619 and U46619+KCl conditions, indicating that the diminished relaxation in KCl containing solution is cGMP-independent mechanism(s). To understand this further, cytosolic $Ca^{2+}$ changes due to NO were compared in rat thoracic aorta by exploiting photoactivated NO using streptozotocin (STZ) that was contracted with either NE or KCl. Fura-3 $[Ca]_{cyt}$ signal caused by NO was small and transient in high $K^+$-, but large and sustained in NE-contracted aorta. The inhibitory potency of STZ expressed in terms of $IC_{50}$ was 5.14 and 3.88 ${\mu}M$ in NE and in high $K^+$, respectively. These results suggest that modification of the cellular mobilization of $Ca^{2+}$ rather than cGMP levels may be an important mechanism for the NO-mediated relaxation when vascular membrane is depolarized, such as atherosclerosis and hypertension.

• The Korean Journal of Physiology
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• v.28 no.2
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• pp.181-190
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• 1994

Endothelium-derived relaxing factor (EDRF) activates guanylate cyclase which mediates the formation of cGMP from GTP in vascular smooth muscle. It is well known that endothelium-dependent relaxation is impaired in spontaneously hypertensive rats (SHR). However, it is still unknown whether the impaired endothelium-dependent relaxation in SHR results from the reduced release of EDRF or from the decrease of vascular response to EDRF. We investigated the effects of cGMP on the contractility and Ca movement in the aorta of SHR and Wistar-Kyoto rats (WKY). The amplitude of the endothelium-dependent relaxation to actylcholine (ACh) was significantly less in SHR than in WKY. L-arginine $(10^{-3}M)$ did not increase endothelium-dependent relaxation in both strains. Sodium nitroprusside (SNP), an activator of guanylate cyclase, relaxed the 40 mM $K^+-induced$ contraction in a dose-dependent manner $(10^{-10}{\sim}10^{-6}\;M)$ in the endothelium-rubbed aortic strips of both strains. However, there was no significant difference in these relaxations between WKY and SHR. 8-bromo-cyclic guanosine monophosphate (8-Br-cGMP), a cell membrane-permeable derivative of cGMP relaxed the 40 mM $K^+-induced$ contraction in a dose-dependent manner $(10^{-6}{\sim}10^{-4}\;M)$ in the endothelium-rubbed aortic strips of both strains. Also norepinephrine $(10^{-6}\;M)-induced$ contractions in normal and Ca-free Tyrode's solution were suppressed by the pretreatment with 8-Br-cGMP $(10^{-4}\;M)$ in either strain. However, the amplitudes of suppression induced by 8-Br-cGMP were greater in SHR than that in WKY. Basal $^{45}Ca$ uptake and 40mM $K^+-stimulated\;^{45}Ca$ uptake were not suppressed by pretreatment with 8-Br-cGMP $(10^{-4}\;M)$ in single aortic smooth muscle cells of both SHR and WKY. From the above results, it is suggested that cGMP decreases Ca sensitivity in vascular smooth muscle cells and that the impaired endothelium-dependent relaxation in the aortic strips of SHR is not the result of a reduced vascular response to EDRF.