Attenuation of ischemia-induced regional myocardial acidosis by LP-805, a newly developed vasodilator, in dogs

TRPV1 Gene Knockout Impairs Postischemic Recovery in Isolated Perfused Heart in Mice

BACKGROUND

Although pharmacological studies suggest that the transient receptor potential vanilloid type 1 (TRPV1) channels expressed in sensory nerve fibers innervating the heart may exert a cardioprotective effect, definitive evidence supporting such a notion is lacking. In addition, function and regulation of sensory neuropeptides, namely, calcitonin gene-related peptide (CGRP) and substance P (SP), in the face of challenges induced by cardiac injury in the presence or absence of the TRPV1 are largely unknown.

METHODS AND RESULTS

The hearts of gene-targeted TRPV1-null mutant (TRPV1(-/-)) mice or wild-type (WT) mice were perfused in a Langendorff apparatus in the presence or absence of capsazepine (a TRPV1 receptor antagonist), CGRP, CGRP(8-37) (a CGRP receptor antagonist), SP, or RP67580 (a neurokinin-1 [NK1] receptor antagonist) when hearts were subjected to 40 minutes of ischemia and 30 minutes of reperfusion. Hemodynamic alterations and SP release measured by radioimmunoassay were assessed before and after ischemia/reperfusion injury of the heart. Expression of the NK1 receptor in the hearts of TRPV1(-/-) and WT mice were determined with the use of Western blot analyses. Impairment of postischemic recovery, defined by increased left ventricular end-diastolic pressure (LVEDP) and decreased left ventricular developed pressure (LVDP) and coronary flow (CF), was more severe in TRPV1(-/-) hearts than in WT hearts. Although it had no effect on postischemic recovery of TRPV1(-/-) hearts, blockade of the TRPV1 with capsazepine caused a most severe impairment of postischemic recovery in WT hearts compared with untreated WT and TRPV1(-/-) hearts. Exogenous CGRP and SP produced a significant improvement in postischemic recovery in both TRPV1(-/-) and WT hearts, and the maximal functional improvement in TRPV1(-/-) hearts was not different from that of WT hearts except that SP-induced increases in LVDP were larger in the former than in the latter. Blockade of the NK1 receptor with RP67580, but not blockade of the CGRP receptor with CGRP(8-37), caused more severe impairment in postischemic recovery in both TRPV1(-/-) and WT hearts than in untreated hearts in both genotypes. The release of SP after ischemia/reperfusion injury was increased in both WT and TRPV1(-/-) hearts, albeit with a smaller magnitude of the increase in the latter. Capsazepine attenuated injury-induced SP release in WT but not TRPV1(-/-) hearts. There was no difference in the expression of the NK1 receptor between the 2 genotype hearts.

CONCLUSIONS

Thus, our data show that (1) TRPV1 gene deletion decreases injury-induced SP release and impairs cardiac recovery function after ischemia/reperfusion injury; (2) TRPV1 gene deletion leads to reconditioning of the heart with improved postischemic recovery compared with that induced by acute TRPV1 blockade and in terms of cardiac response to exogenous SP; and (3) blockade of the NK1 but not CGRP receptors worsens postischemic recovery of hearts in both genotypes. Taken together, these data indicate that TRPV1 plays a role in protecting the heart from injury possibly via increasing SP release and that deletion of this receptor reconditions the heart for escaping, at least in part, from injury possibly via enhancing NK1 receptor function.

Ion channels involved in the release of calcitonin gene-related peptide by low pH, prostacyclin and capsaicin in the isolated guinea-pig heart

The aim of this study was to characterise the release of calcitonin gene-related peptide (CGRP) by capsaicin, low pH and prostacyclin in terms of Ca2+ channel dependence, interactions with K(ATP) channels and the role of action potential propagation, in the isolated, perfused guinea-pig heart. The Ca2+ channel blocker omega-conotoxin reduced CGRP release evoked by 10(-7) M capsaicin, as well as CGRP release evoked by pH 7. CGRP release caused by capsaicin at low (10(-7) M) but not high (10(-6) M) concentrations was also attenuated by tetrodotoxin, indicating partial dependence on action potential propagation. CGRP release caused by prostacyclin was not altered by any of the tested drugs. The K(ATP) channel activator cromakalim and the K(ATP) channel blocker glibenclamide had no effect on CGRP release. Previous findings that low pH and capsaicin stimulate capsaicin-sensitive afferents in the isolated heart at least partly through common mechanisms are thus supported. Attenuation of capsaicin-evoked release of CGRP by tetrodotoxin suggests recruitment of additional nerve terminals by a local axon reflex.

Attenuation of low pH-, but not capsaicin- or PGI2-evoked CGRP-release by endothelium removal using saponin

The aim of the present study was to evaluate the role of the endothelium in low pH-, capsaicin-, and prostacyclin (PGI2)-evoked release of calcitonin gene-related peptide (CGRP)-like immunoreactivity (-LI) from C-fibre afferents in the isolated, perfused guinea-pig heart. CGRP-LI release, and formation of the stable PGI2-metabolite 6-keto-PGF1 alpha, in response to moderate acidosis (pH 7, 6, but not 5) were significantly reduced after removal of endothelium using saponin (50 micrograms mL-1) perfusion. In contrast, the release of CGRP-LI evoked by capsaicin (10(-7) M) or PGI2 (10(-5) M) remained unchanged after removal of the endothelium. Saponin treatment did not influence the vasodilator action of CGRP, whereas the vasodilation evoked by substance P (SP) was abolished. It is concluded that CGRP release evoked by low pH, but not that evoked by capsaicin or exogenous PGI2, is partly endothelium dependent. Our data suggest that endothelially produced PGI2 is involved in low pH-evoked release of CGRP from capsaicin sensitive nerves in the heart.

Capsazepine-sensitive release of calcitonin gene-related peptide from C-fibre afferents in the guinea-pig heart by low pH and lactic acid

The present study aimed to evaluate the possible influence of the selective capsaicin antagonist, capsazepine, on the release of calcitonin gene-related peptide (CGRP)-like immunoreactivity from sensory nerves in the isolated perfused guinea-pig heart. Low-pH buffer (pH 7, 6, 5), capsaicin (10(-7) M), lactic acid (5, 20, 50 mM) and nicotine (10(-4) M) all evoked a clear-cut release of CGRP-like immunoreactivity. Incubation with capsazepine (10(-6) to 10(-5) M) significantly reduced the CGRP-like immunoreactivity release evoked by low pH, capsaicin and lactic acid (5 mM) but not that evoked by nicotine. Furthermore, the capsaicin-evoked stimulation of heart rate was inhibited by incubation with capsazepine. The inorganic dye, ruthenium red, which has previously been shown to attenuate capsaicin-, but not nicotine-induced CGRP release from the heart, also reduced the release of CGRP caused by low pH and lactic acid (5 mM). It is concluded that the CGRP-like immunoreactivity release evoked from the heart by low pH and lactic acid shares several characteristic features with the release evoked by capsaicin. Since tissue pH is low in myocardial ischaemia and this is well known to cause pain, the use of capsazepine to inhibit the function of C-fibre afferents may represent a novel principle to influence autonomic reflex reactions associated with cardiac pathological conditions.

Effects of LP-805, a newly developed vasodilator, on myocardial metabolism in ischaemic dog hearts

The effects of LP-805, a newly developed vasodilator, on changes in the myocardial energy and carbohydrate metabolism induced by ischaemia were studied in open-chest anaesthetized dogs. Ischaemia was induced by ligating the left anterior descending coronary artery for 3 min. The myocardial energy stores were depleted, and the levels of glycolytic intermediates were altered 3 min after the onset of ischaemia. Energy change potential was decreased, and ([G6P] + [F6P])/[FDP] and [lactate]/[pyruvate] ratios were increased by ischaemia. These findings indicated that the myocardial metabolism was converted from an aerobic to an anaerobic type by ischaemia. LP-805 (10, 30, or 100 micrograms kg-1) was injected intravenously 5 min before the onset of ischaemia. LP-805 prevented the myocardial energy depletion and alterations of myocardial carbohydrate metabolism due to ischaemia, indicating that it appeared to convert the anaerobic metabolism back to aerobic metabolism in the ischaemic myocardium. In conclusion, LP-805 may reduce the ischaemic influence on the myocardium.

Capsazepine inhibits low pH- and lactic acid-evoked release of calcitonin gene-related peptide from sensory nerves in guinea-pig heart

In the isolated perfused guinea-pig heart low-pH buffer (pH 5), lactic acid (5 mM), capsaicin (10(-7) M) and nicotine (10(-4) M) all evoked a clear-cut release of calcitonin gene-related peptide (CGRP) from sensory nerves. Incubation with the proposed capsaicin receptor antagonist, capsazepine (10(-5) M), significantly reduced the CGRP release evoked not only by capsaicin but also by low pH and lactic acid, indicating a common mechanism of C-fibre activation, while the nicotine response remained unchanged.