Identifying the Source of a Humoral Factor of Remote (Pre)Conditioning Cardioprotection

Abstract

Signalling pathways underlying the phenomenon of remote ischaemic preconditioning (RPc) cardioprotection are not completely understood. The existing evidence agrees that intact sensory innervation of the remote tissue/organ is required for the release into the systemic circulation of preconditioning factor(s) capable of protecting a transplanted or isolated heart. However, the source and molecular identities of these factors remain unknown. Since the efficacy of RPc cardioprotection is critically dependent upon vagal activity and muscarinic mechanisms, we hypothesized that the humoral RPc factor is produced by the internal organ(s), which receive rich parasympathetic innervation. In a rat model of myocardial ischaemia/reperfusion injury we determined the efficacy of limb RPc in establishing cardioprotection after denervation of various visceral organs by sectioning celiac, hepatic, anterior and posterior gastric branches of the vagus nerve. Electrical stimulation was applied to individually sectioned branches to determine whether enhanced vagal input to a particular target area is sufficient to establish cardioprotection. It was found that RPc cardioprotection is abolished in conditions of either total subdiaphragmatic vagotomy, gastric vagotomy or sectioning of the posterior gastric branch. The efficacy of RPc cardioprotection was preserved when hepatic, celiac or anterior gastric vagal branches were cut. In the absence of remote ischaemia/reperfusion, electrical stimulation of the posterior gastric branch reduced infarct size, mimicking the effect of RPc. These data suggest that the circulating factor (or factors) of RPc are produced and released into the systemic circulation by the visceral organ(s) innervated by the posterior gastric branch of the vagus nerve.

Fig 1. Experimental interventions.

Previous studies—DVMN silencing, cervical vagotomy. Six types of subdiaphragmatic vagotomy performed in the current study: total, bilateral gastric, anterior gastric, posterior gastric, hepatic and celiac, are shown on a schematic representation of typical distribution of rat abdominal vagal branches. Agb, anterior gastric branch; Avt, anterior vagal trunk; Ccb, common celiac branch; Hb, hepatic branch; Lvn, left vagus nerve; Pgb, posterior gastric branch; Pvt, posterior vagal trunk; Rvn, right vagus nerve. Brain, lungs, heart, diaphragm, liver, stomach, pancreas, small intestine and colon are depicted schematically.

Fig 2. Cardioprotection established by remote ischaemic preconditioning (RPc) requires intact parasympathetic innervation of visceral organs.

(a) Illustration of the experimental protocols. RPc was induced by 15 min occlusion of both femoral arteries, followed by 10 min reperfusion. Sham-RPc procedure involved dissection of both femoral arteries without occlusion. Arrows indicate time of total subdiaphragmatic vagotomy, selective sectioning of individual visceral branches or sham surgery. (b) Total subdiaphragmatic vagotomy, bilateral gastric vagotomy and selective sectioning of the posterior gastric branch abolished the cardioprotective effect of RPc, whereas sectioning of the anterior gastric, celiac or hepatic branches had no effect on RPc cardioprotection. The infarct size is presented as the percentage of the area at risk. Individual data and means ± SEM are shown. P-values correspond to the Dunn’s post-hoc tests.

Fig 3. Electrical stimulation of the posterior gastric vagal branch mimics RPc cardioprotection.

(a) Illustration of the experimental protocols. Electrical stimulation (stim.) of individual vagal branches commenced 25 min before the onset of myocardial ischaemia (MI) and continued 10 min into the period of reperfusion. Sham procedure involved surgical dissection of the nerve and placing it on the electrodes without stimulation. (b) Electrical stimulation of the posterior gastric vagal branch reduced the extent of myocardial ischaemia/reperfusion injury, whereas stimulation of the hepatic vagal branch or sham stimulation of the posterior gastric branch had no effect. The infarct size is presented as the percentage of area at risk. Individual data and means ± SEM are shown. P-values correspond to the Dunn’s post-hoc tests.

Fig 4. Diagrammatic representation of the nervous control of hormone secretion by enteroendocrine cells of the gastrointestinal tract.

Extrinsic vagal parasympathetic nerves either directly or via activation of the enteric neurones trigger release of hormones (hypothesised circulating cardioprotective factors) by releasing acetylcholine (among other transmitters). ACh, acetylcholine; AChR, acetylcholine receptor; BOM, bombesin; CCK, cholecystokinin; CGPR, calcitonin gene-related peptide; GliC, glicentin; GLP-1/2, glucagon-like peptide-1 and 2; OXM, oxyntomodulin; PYY, peptide YY; VIP, vasoactive intestinal peptide.