Role of protons in activation of cardiac sympathetic C-fibre afferents during ischaemia in cats

Abstract

1. Chest pain caused by myocardial ischaemia is mediated by cardiac sympathetic afferents. The mechanisms of activation of cardiac afferents during ischaemia remain poorly understood. Increased lactic acid production is associated closely with myocardial ischaemia. The present study examined the role of protons generated during ischaemia in activation of cardiac sympathetic C-fibre afferents. 2. Single-unit activity of cardiac afferents innervating both ventricles was recorded from the left sympathetic chain in anaesthetized cats. Epicardial tissue pH was measured within 1-1.5 mm of the surface by a pH-sensitive needle electrode. Responses of cardiac afferents to myocardial ischaemia, lactic acid, sodium lactate, acidic phosphate buffer and hypercapnia were determined. 3. Occlusion of the coronary artery for 5 min decreased epicardial tissue pH from 7.35 +/- 0.21 to 6.98 +/- 0.22 (P < 0.05). Epicardial placement of isotonic neutral phosphate buffer, but not saline, prevented the ischaemia-induced decrease in epicardial pH. This manoeuvre significantly attenuated the response of 16 afferents to 5 min of ischaemia (1.56 +/- 0.23 pre-treatment vs. 0.67 +/- 0.18 impulses s-1). Topical application of 10-100 microg ml-1 of lactic acid, but not sodium lactate, concentration-dependently stimulated 18 cardiac afferents. Inhalation with high-CO2 gas failed to activate 12 separate cardiac afferents. Furthermore, lactic acid stimulated cardiac afferents to a greater extent than acidic phosphate buffer solution, applied at a similar pH to the same afferents. 4. Collectively, this study provides important in vivo evidence that protons contribute to activation/sensitization of cardiac sympathetic C-fibre afferents during myocardial ischaemia.

Figure 1. Effect of isotonic neutral buffer on changes in epicardial pH induced by myocardial ischaemia

Original record showing the epicardial pH changes during 5 min of myocardial ischaemia in the presence of saline (A) and isotonic neutral phosphate buffer (B) in one animal.

Figure 2. Bar graph summarizing changes in epicardial pH during control and 5 min of ischaemia in the presence of saline and phosphate buffer

Columns and error bars represent means ±s.e.m.*P < 0.05 compared with pre-ischaemia control.

Figure 3. Original representative tracings showing responses of an ischaemically sensitive cardiac afferent to topical application (↑) of lactic acid or sodium lactate

The afferent ending was located in the anterior left ventricle and had a conduction velocity of 0.46 m s⁻¹. During topical applications of lactic acid at 20, 50 and 100 μg ml⁻¹ and of sodium lactate at 100 μg ml⁻¹, the epicardial pH values measured by the tissue electrode were 7.18, 7.03, 6.83 and 7.34, respectively.

Figure 4. Concentration-dependent responses of 18 ischaemically sensitive cardiac afferents to topical (epicardial) application of lactic acid

Data are presented as means ±s.e.m.*P < 0.05 compared with the discharge activity during control. During topical applications of lactic acid at 20, 50 and 100 μg ml⁻¹, the epicardial pH values were 7.20 ± 0.04, 7.00 ± 0.04 and 6.80 ± 0.06, respectively.

Figure 5. Bar graph showing responses of 12 ischaemically sensitive cardiac afferents to 5 min of ischaemia and inhalation of high-CO₂ gas

Columns and error bars represent means ±s.e.m.*P < 0.05 compared with pre-ischaemia control. Tissue pH values during ischaemia and hypercapnia were 6.98 ± 0.21 and 6.96 ± 0.21, respectively.

Figure 6. Bar graph showing responses of 8 ischaemically sensitive cardiac afferents to topical application of lactic acid or isotonic phosphate buffer

With both lactic acid (50 μg ml⁻¹), and isotonic phosphate buffer, pH = 5.42. Columns and error bars represent means ±s.e.m.*P < 0.05 compared with the afferent activity during control. **P < 0.05 compared with afferent response to lactic acid. The epicardial interstitial pH was 7.0 ± 0.08 during topical application of lactic acid and 6.9 ± 0.08 during application of acidic phosphate buffer.

Figure 7. Representative histograms showing the discharge activity of a cardiac afferent during control, ischaemia and reperfusion before (A) and after (B) treatment with isotonic neutral phosphate buffer

The afferent ending was located in the anterior left ventricle and had a conduction velocity of 0.64 m s⁻¹. Traces 1 and 2: original tracings of this afferent recorded at the times indicated by bars above histograms.

Figure 8. Bar graphs showing the response of cardiac sympathetic afferents to repeated 5 min periods of ischaemia in the absence and presence of isotonic neutral phosphate buffer (A) or saline (B)

Columns and error bars represent means ±s.e.m.*P < 0.05 compared with respective pre-ischaemic control. **P < 0.05 compared with the initial afferent response to ischaemia.