ET(B)-mediated contraction differs between left descending coronary artery and its next branch

Sodium-Calcium Exchanger in Pig Coronary Artery

This review focuses on the sodium-calcium exchangers (NCX) in the left anterior descending coronary artery smooth muscle. Bathing tissues in Na⁺-substituted solutions caused them to contract. In cultured smooth muscle cells, it increased the cytosolic Ca²⁺ concentration and extracellular entry of ⁴⁵Ca²⁺. All three activities were attributed to NCX since they were inhibited by NCX inhibitors. The tissues also expressed the sarco/endoplasmic reticulum (SER) Ca²⁺ pump SERCA2b whose activity was much greater than that of NCX. Inhibiting SERCA2b with thapsigargin decreased the NCX-mediated ⁴⁵Ca²⁺ accumulation by the cells. The decrease was not observed in cells loaded with the Ca²⁺-chelator BAPTA. The results are consistent with a limited diffusional space model with a proximity between NCX and SERCA2b. NCX molecules appear to be colocalized with the subsarcolemmal SERCA2b based on studies on membrane flotation experiments and microscopic fluorescence imaging of antibody-labeled cells. Thapsigargin inhibition of SERCA2b moved NCX even closer to SER. This provides a model for the NCX-mediated Ca²⁺ refilling of SER in the arterial smooth muscle. The model for the NCX-mediated refilling of the depleted SER proposed for smooth muscle did not apply to endothelium in which NCX levels were greater and SERCA levels were lower than in smooth muscle. The effect of thapsigargin on the NCX-mediated Ca²⁺ accumulation which was observed in smooth muscle was absent in the endothelium. We propose that the coupling between NCX and smooth muscle may be tissue dependent.

Permanent distal occlusion of middle cerebral artery in rat causes local increased ETB, 5-HT₁B and AT₁ receptor-mediated contractility downstream of occlusion

BACKGROUND/AIMS

In response to experimental stroke, a characteristic functional and expressional upregulation of contractile G-protein-coupled receptors has been uncovered in the affected cerebral vasculature; however, the mechanism initiating this phenomenon remains unknown.

METHODS

Using a model of permanent distal occlusion of rat middle cerebral arteries, we investigated whether there was a regional difference in receptor-mediated contractility of segments located upstream and downstream of the occlusion site. The contractile response to endothelin, angiotensin and 5-hydroxytryptamine receptor stimulation was studied by sensitive wire myograph.

RESULTS

Only downstream segments exhibited an augmented contractile response to stimulation with each of the three ligands, with the response towards sarafotoxin 6c being especially augmented compared to sham, upstream and contralateral controls. This functional increase did not seem to relate to ischemic tissue damage, inflammatory cell infiltration or the element of reperfusion. Interestingly, immunohistochemistry did not show any difference in the level of immunoreactivity towards endothelin B (ETB) receptors between groups.

CONCLUSION

Single artery occlusion without significant visible infarct resulted in locally increased ETB, angiotensin type 1 and 5-hydroxytryptamine 1B receptor-mediated contractile responses only in segments located downstream of the occlusion site. This suggests lack of wall stress as an initiating trigger leading to regulation of contractile response after cerebral stroke.

Sodium-calcium exchange mediated contraction in left anterior descending and left ventricular branch arteries

We tested the hypothesis that the de-endothelialized artery rings from the left anterior descending (LAD) coronary artery and its left ventricular branch (LVB) differ in their contractile responses to Na⁺–Ca²⁺-exchanger (NCX) mediated Ca²⁺-entry, muscarinic receptor activation with carbachol, and sarco/endoplasmic reticulum Ca²⁺ pump (SERCA) inhibition with thapsigargin. In LVB, the force of contraction (in N/g tissue) produced by the NCX mediated Ca²⁺-entry (17.5 ± 1.4) and carbachol (18 ± 1.5) was only slightly smaller than that due to membrane depolarization with KCl (24.0 ± 1.0). In contrast, in LAD the force of contraction produced with NCX (8.7 ± 0.7) and carbachol (6.1 ± 1.1) was much smaller than with KCl (15.7 ± 0.7). Thapsigargin also contracted LVB with greater force than LAD. When isolated microsomes were used, the binding to the muscarinic receptor antagonist quinuclidinyl benzilate was greater in LVB than in LAD. Microsomes were also used for Western blots. The intensities of signals for both SERCA and NCX were greater in LVB than in LAD. These biochemical observations were consistent with the contractile experiments. Thus, it appears that the differences between LAD and the resistance arteries may begin as early as LVB.

Differences in endothelin receptor types in the vasculature of Bothrops jararaca (Viperidae) and Oxyrhopus guibei (Colubridae) snakes

Endothelins (ETs) are vasoactive peptides evolutionary well conserved that exert their effects through two specific receptors (ET(A) and ET(B)) widely distributed in all vertebrates. In snakes, the presence and function of endothelins and their receptors are still scarcely described. We have recently demonstrated the presence of ET(A) and ET(B2) receptors in the snake Bothrops jararaca (Bj). In the present work we showed that distinctively from Bj, the vascular contraction induced by endothelin in Oxyrhopus guibei (Og) snake is mediated only by ET(A) receptors. Selective ET(B) agonists (SRTX-c and IRL(1620)) and antagonists (IRL(1038) and BQ(788)) were ineffective in Og preparations of isolated aorta. We also showed that ET-1 response on Og arterial blood pressure was monophasic hypertensive as opposed to biphasic (hypotension followed by hypertension) in Bj. Furthermore, we characterized the relaxing properties of endothelin receptor ET(B1) in pre-contracted aorta preparations. We showed that IRL(1620) induced relaxation of pre-contracted Bj aorta but was ineffective in relaxing Og preparations. IRL(1620) relaxing effect on Bj aorta was abolished by l-NAME, indicating involvement of NO release, and was reduced by selective ET(B) antagonists. Our findings suggest that Og snake has a more primitive spectrum of ET receptors (only ET(A) receptor) than Bj (presence of ET(A), ET(B1) and ET(B2) receptors).