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

Stimulated release of calcitonin gene-related peptide from the human right atrium in patients with and without diabetes mellitus

Calcitonin gene-related peptide (CGRP), a potent vasodilator released during activation from a subset of sensory Adelta- and C-fiber afferents, has been suggested to play a beneficial role in myocardial ischemia. Variations in CGRP release can possibly be correlated with diseases that involve changes in activity or degeneration of cardiac afferent fibers. The aim of the present study was to examine basal and stimulated CGRP release from cardiac tissue in patients who underwent cardiopulmonary bypass surgery and to compare patients with and without known history of diabetes mellitus. Small pieces of the right atrium routinely excised during the bypass operations were passed through series of oxygenated solutions. The TRPV1 receptor agonist capsaicin and the nitric oxide donor NONOate were added for stimulation of cardiac afferent fibers. The eluates were processed using an enzyme immuno-assay (EIA) for measurement of CGRP concentrations. Both capsaicin and NONOate caused significant increases in CGRP release. No significant differences in CGRP release between patients with and without diabetes mellitus were examined. The present study evaluates a simple and reproducible model for measuring stimulated CGRP release from the human right atrium.

Release of calcitonin gene-related peptide from the isolated mouse heart: methodological validation of a new model

Calcitonin gene-related peptide (CGRP), a potent vasodilator released from a subset of sensory Adelta- and C-fiber afferents, has been suggested to play a beneficial role in myocardial ischemia. Variations in CGRP release can possibly be correlated with diseases that involve changes in activity or degeneration of cardial afferent fibers. The aim of the present study was to evaluate a simple and easily reproducible model for measuring stimulated CGRP release from cardial afferents. For this reason freshly isolated mouse hearts were passed through incubations in series of 25 min. Solutions consisting of oxygenated synthetic interstitial fluid. Dissolved substances such as capsaicin, bradykinin and potassium chloride were used as excitatory test stimuli. For comparison, isolated mouse hearts were perfused with the same solutions through a cannula inserted into the rising aorta. The eluates were processed using an enzyme immuno-assay for measurement of CGRP concentrations. Capsaicin, bradykinin and the potassium solution caused concentration-dependent increases in CGRP release. There were no differences in CGRP release when oxygen was replaced by nitrogen in the solutions. Immersion of hearts caused significantly more CGRP release than perfusion with same solutions. The results suggest that mouse heart immersion is more effective than coronary perfusion in measuring stimulated CGRP release from cardial afferents which are widely distributed in the epicardium but rare in deeper myocardial layers. The results are discussed with respect to the potentially important vasodilatory and cardioprotective functions of CGRP released from activated epicardial afferents.

Cellular mechanisms underlying cutaneous pressure-induced vasodilation: in vivo involvement of potassium channels

In the skin of humans and rodents, local pressure induces localized cutaneous vasodilation, which may be protective against pressure-induced microvascular dysfunction and lesion formation. Once activated by the local pressure application, capsaicin-sensitive nerve fibers release neuropeptides that act on the endothelium to synthesize and release nitric oxide (NO) and prostaglandins, leading to the development of the cutaneous pressure-induced vasodilation (PIV). The present study was undertaken to test in vivo the hypothesis that PIV is mediated or modulated by differential activation of K+ channels in anesthetized rats using pharmacological methods. Local pressure was applied at 11.1 Pa/s. Endothelium-independent and -dependent vasodilation were tested using iontophoretic delivery of sodium nitroprusside (SNP) and acetylcholine (ACh), respectively, and was correlated with PIV response. PIV was reduced after systemic administration of tetraethylammonium (a nonspecific K+ channel blocker), iberiotoxin [a specific large-conductance Ca2+-activated K+ (BKCa) channel blocker], and glibenclamide [a specific ATP-sensitive K+ (KATP) channel blocker], whereas PIV was unchanged by apamin (a specific small-conductance Ca2+-activated K+ channel blocker) and 4-aminopyridine (a specific voltage-sensitive K+ channel blocker). The responses to SNP and ACh were reduced by iberiotoxin but were unchanged by glibenclamide. We conclude that the cellular mechanism of PIV in skin involves BKCa and KATP channels. We suggest that the opening of BKCa and KATP channels contributes to the hyperpolarization of vascular smooth muscle cells to produce PIV development mainly via the NO and prostaglandin pathways, respectively.

In vitro selection of a peptide with high selectivity for cardiomyocytes in vivo

One approach to targeted therapies for cardiovascular disease relies on isolating ligands that enhance the tissue-specific uptake of genes or drugs by heart cells. To obtain heart-targeting ligands, phage display biopanning was used to isolate a 20-mer peptide that binds to isolated primary cardiomyocytes. The isolated phage, PCM.1, displays the peptide WLSEAGPVVTVRALRGTGSW, and binds these cells 180 times better than a control phage from the library. Furthermore, phage displaying this peptide preferentially bind to cardiomyocytes when compared with a panel of other cell types. A BLAST search revealed that this peptide contains a 12 amino acid segment with sequence identity to a peptide in tenascin-X, an extracellular matrix protein. Synthetic peptides containing the complete 20-mer or a 12-mer tenascin peptide partially blocked phage binding to the cardiomyocytes. We developed a quantitative real-time PCR assay to assess uptake of this phage by tissues in vivo. Using this assay, preferential localization of the PCM.1 phage in heart was observed compared to the uptake of this phage by other tissues or other phage by heart. Furthermore, PCM.1 phage was associated with cardiomyocytes isolated from mice treated with a phage in vivo. These results demonstrate the utility of biopanning on isolated cells for identifying specific binding peptides that can target a tissue in vivo.

Changes in extracellular pH and myocardial ischaemia alter the cardiac effects of diadenosine tetraphosphate and pentaphosphate

1. The structural conformation of diadenosine tetraphosphate (Ap(4)A) and pentaphosphate (Ap(5)A) has been reported to alter as pH is reduced. As such, it is possible that the cardiac effects of Ap(4)A and Ap(5)A vary during acidosis and myocardial ischaemia due to changes in ligand structure, receptor proteins or intracellular signalling. 2. We investigated whether the cardiac electrophysiological and coronary vasomotor effects of Ap(4)A and Ap(5)A are preserved under conditions of extracellular acidosis (pH 6.5) and alkalosis (pH 8.5) and whether Ap(4)A has any electrophysiological or antiarrhythmic effects during ischaemia. 3. Transmembrane right ventricular action potentials, refractory periods and coronary perfusion pressure were recorded from isolated, Langendorff-perfused guinea-pig hearts under constant flow conditions. The effects of 1 nM and 1 microM Ap(4)A and Ap(5)A were studied at pH 7.4, 6.5 and 8.5. The effects of 1 microM Ap(4)A were studied during global low-flow ischaemia and reperfusion. 4. At pH 7.4, Ap(4)A and Ap(5)A increased action potential duration (APD(95)) and refractory period (RP) and reduced coronary perfusion pressure. The electrophysiological effects were absent at pH 6.5 while the reductions in perfusion pressure were attenuated. At pH 8.5, Ap(4)A increased RP but the effects of Ap(4)A and Ap(5)A on perfusion pressure were attenuated. During ischaemia, Ap(4)A had no antiarrhythmic or electrophysiological effects. 5. These data demonstrate the importance of extracellular pH in influencing the effects of Ap(4)A and Ap(5)A on the heart and indicate that any potentially cardioprotective effects of these compounds during normal perfusion at physiological pH are absent during ischaemia.

Pharmacological evidence for CGRP uptake into perivascular capsaicin sensitive nerve terminals

Specific mechanisms, providing reuptake of cathecholamine and amino acid neurotransmitters (e.g. serotonin and glutamate) into cells of the central nervous system are well known, whereas neuronal uptake of neuropeptide transmitters have not previously been reported. In the present study we present evidence for uptake of the 37 amino acid neuropeptide, calcitonin gene-related peptide (CGRP) into perivascular terminals of capsaicin sensitive nerve fibres, innervating the guinea-pig basilar artery. Release of CGRP from perivascular nerve terminals was obtained by capsaicin-induced vanilloid receptor-stimulation and detected as CGRP receptor-mediated dilation of isolated segments of the guinea-pig basilar artery. Following three repeated capsaicin challenges, CGRP-depleted segments were incubated with CGRP. This caused significant reappearance of capsaicin-induced vasodilatory responses. These responses were dependent on duration and concentration of the preceding CGRP incubation and were inhibited by the CGRP receptor antagonist, CGRP(8 - 37). The CGRP-re-depletion was significantly reduced when CGRP(8 - 37) was present during the preceding CGRP incubation. Thus, presynaptic CGRP receptors are likely to be involved in neuronal CGRP uptake. Incubating the artery segments with (125)I-CGRP allowed subsequent detection of capsaicin-induced (125)I-release. Immunohistochemical experiments showed that only terminal CGRP is subject to capsaicin-induced depletion in vitro, whereas CGRP-immunoreactivity endures in the nerve fibres.

Capsaicin-sensitive mechanisms are involved in cortical spreading depression-associated cerebral blood flow changes in rats

We tested the hypothesis that capsaicin-sensitive mechanisms play a role in the cortical spreading depression (CSD)-related changes in cortical blood flow (CBF). CBF was measured with laser Doppler flowmetry in anesthetized rats. The animals were treated with capsaicin before (48 h-2 weeks) or during the experiments. This agent is thought to stimulate small-diameter sensory nerve fibers selectively and to deplete stored peptides. In the vehicle-treated group (n=8), the peak value of the CSD-associated hyperperfusion was 257+/-12% above the baseline (mean+/-SEM, P<0.05). In the groups treated with 20 and 40 microg/kg or 20 mg/kg capsaicin, there were only small decreases in CBF. In the groups treated with 100 mg/kg capsaicin, the CSD-associated hyperemia was reduced at 48 h (158+/-7%, P<0.05). However, at 96 h a transient hyperresponsiveness (390+/-38%, P<0.05) was observed, which had disappeared by 2 weeks. These results indicate that the manipulation of sensory neuropeptide stores results in a biphasic effect on CSD-induced CBF responses.