Local cardiac effects of substance P and its role in myocardial dysfunction during ischemia and reperfusion

Abstract

Substance P (SP) is widely distributed in mammalian hearts. It can be released by different chemicals and during path~logical conditions. These facts are consistent with the opinion that SP in heart has physiological or pathological ' relevance. This study was designed to investigate the cardiac effects of SP and the mechanisms involved in its actions. The role of SPin myocardial dysfunction during myocardial ischemia/ reperfusion was also investigated. Local cardiac effects ~f SP were studied in isolated guinea pig heart and atrial preparations. In heart and right atrial preparations, SP caused negative changes in rate and force. Both chronotropic and inotropic changes were dose- or concentration- related. In electrically stimulated left atrial preparations, SP had no significant effect on contractile force at concentration up to 3x10-5 M. A series of experiments was conducted to understand the mechanisms involved in the actions ofSP~ It was hypothesized that cholinergic neurons modu1ate the effects of SP. Atropine, a muscarinic receptor antagonlst, at a concentration of 1 o- 6 M significantly blocked the negative rate and force effects of SPin both heart and atrial preparations. Depletion of acetylcholine (ACh), achieved by concomitant el~ctrical stimulation of the vagus nerves and perfusion with hemicholinium-3, also significantly antagonized the negative effects ofSP. There was no significant effect of SP observed in hearts with ACh depletion or with muscarinic receptor blockade. These results suggest that SP elicits its effects mainly through the release of ACh. The role of adrenergic neurons in the effects of SP was evaluated by use of adrenergic receptor blocka~~ and norepinephrine (NE) depletion. Phentolamine and nadolol (a- and J3--adrenergic receptor antagonists, respectively) at a concentration oflQ-6 M significantly blocked the rate and force effects of isoprotenolol but did not affect the actions of SP. This finding suggests that SP does not affect adrenergic neurons. However,_ depletion ofNE by reserpine pretreatment significantly potentiated both rate and force effects of SP. One explanation for this observation could be that reserpine exerts a non-selective supersensitivity within the tissue. Our observation that reserpine pretreatment increased the sensitivity of muscarinic receptors to ACh appears to support this explanation. Nitric oxide (NO) can be synthesized in the heart and may be involved in the negative rate and force effe_cts mediated by or produced through cholinergic neurons. Many actions of SPin other tissues have been reported to be mediated by NO. Therefore, we hypothesized that NO formation in heart mediates the effects of SP. Nitric oxide is fortl;led from L-arginine by NO synthase (NOS) and elicits its effects by stimulating soiuble guanylyl cyclase (GC) and the formation of cGMP. We utilized NG-nitro L-arginine methyl ester (L-NAME, a NOS inhibitor) or methylene blue (a soluble GC inhibitor) to block the actions of endogenous NO. Methylene blue and L-NAME significantly attenuated the rate and force effects of SP. Subsequent addition of L-arginine reversed the blocking effects of L-NAME. These data suggest that NO mediates the cardiac effects of SP. Post-ischemic ventricular dysfunction (myocardial ~tunning) has been characterized by an impairment of myocardial contraction during reperfusion after a short ischemic period. Due to its negative actions in the heart and a report that SP release is iricreased during myocardial hypoxia, we, hypothesized that SP may mediate this post-ischemic cardiac contractile dysfunction. Our model was isolated guinea pig hearts su~jected to15 minutes of global ischemia and followed by 60 min of reperfusion. The hearts.~xhibited a recovery in left ventricular developed pressure (LVDP) to 54% of the pre-ischemic baseline. After pretreatment of the heart preparations with neurokinin-1 (NKl) receptor antagonists, CP-99,994-01 or span tide, LVDP recovered to 78% and 88% of the pre-ischemic baseline, respectively. Depletion of SP by pretreatment with: capsaicin also significantly enhanced recovery of the LVDP. These results suggest that SP is a mediator of post-ischemi~ myocardial dysfunction. In summary, SPhas both negative inotropic and chronotropic effects in heart. The release of ACh mediates the effects of SP. Nitric oxide also mediates-the effects of SP. Substance.P can be·~eleased in heart during ischemia and reperfusion and appears to play a role in :post-ischeinic cardiac contractile dysfunction.