Voltage-Sensitive K(+) Channels Inhibit Parasympathetic Ganglion Transmission and Vagal Control of Heart Rate in Hypertensive Rats

Abstract

Parasympathetic withdrawal plays an important role in the autonomic dysfunctions in hypertension. Since hyperpolarizing, voltage-sensitive K(+) channels (K V) hamper transmitter release, elevated K V-activity may explain the disturbed vagal control of heart rate (HR) in hypertension. Here, the K V inhibitor 3,4-diaminopyridine was used to demonstrate the impact of K V on autonomic HR control. Cardiac output and HR were recorded by a flow probe on the ascending aorta in anesthetized, normotensive (WKY), and spontaneously hypertensive rats (SHR), and blood pressure by a femoral artery catheter. 3,4-diaminopyridine induced an initial bradycardia, which was greater in SHR than in WKY, followed by sustained tachycardia in both strains. The initial bradycardia was eliminated by acetylcholine synthesis inhibitor (hemicholinium-3) and nicotinic receptor antagonist/ganglion blocker (hexamethonium), and reversed to tachycardia by muscarinic receptor (mAchR) antagonist (atropine). The latter was abolished by sympatho-inhibition (reserpine). Reserpine also eliminated the late, 3,4-diaminopyridine-induced tachycardia in WKY, but induced a sustained atropine-sensitive bradycardia in SHR. Inhibition of the parasympathetic component with hemicholinium-3, hexamethonium, or atropine enhanced the late tachycardia in SHR, whereas hexamethonium reduced the tachycardia in WKY. In conclusion, 3,4-diaminopyridine-induced acetylcholine release, and thus enhanced parasympathetic ganglion transmission, with subsequent mAchR activation and bradycardia. 3,4-diaminopyridine also activated tachycardia, initially by enhancing sympathetic ganglion transmission, subsequently by activation of norepinephrine release from sympathetic nerve terminals. The 3,4-diaminopyridine-induced parasympathetic activation was stronger and more sustained in SHR, demonstrating an enhanced inhibitory control of K V on parasympathetic ganglion transmission. This enhanced K V activity may explain the dysfunctional vagal HR control in SHR.

Keywords: 3,4-diaminopyridine; acetylcholine release; heart rate; hypertension; norepinephrine release; parasympathetic ganglia; sympathetic ganglia; voltage-sensitive K+-channels.

Figure 1

Suggested localization of the inhibitory effect of 3,4-DAP on K_V in pre- and postganglionic parasympathetic and sympathetic neurons involved in the regulation of HR. By inhibiting K_V, 3,4-DAP will induce Ach release from preganglionic parasympathetic and sympathetic nerve endings as well as in postganglionic parasympathetic nerve terminals, and also induce norepinephrine (NE) release from postganglionic sympathetic nerve endings. The inhibitors used to localize the effect of 3,4-DAP, i.e., the Ach synthesis inhibitor hemicholinium-3, the mAchR and nAchR blockers atropine and hexamethonium, respectively, reserpine which depletes sympathetic nerve endings of NE, and the βAR antagonist nadolol, which block sympathetic control of HR, are indicated in capital letters next to site of action. When effect of one drug involved more than one step in the response chain or involved both the parasympathetic and sympathetic chains, drugs were combined to identify the site of 3,4-DAP action. Through the effect on HR, the results showed that Ach release from preganglionic vagal nerves was blunted by augmented 3,4-DAP-sensitive hyperpolarizing K_V activity in SHR (indicated in upper gray box), thus preventing activation of the postganglionic neuron and vagal inhibition of HR (indicated in lower gray box). Presynaptic K_V in the parasympathetic ganglia therefore functioned as a bottleneck in vagal transmission in SHR. These ganglia are likely to be located in superficial fat tissue close to the sinoatrial node. Blunted arrows indicate the inhibitory effect of presynaptic K_V on transmitter release.

Figure 2

The HR response to 3,4-DAP after pre-treatment with cholinergic inhibitors or quinidine. Quinidine was given to SHR only. Significant responses (* within symbol) and group differences (* in brackets) at HR-nadir (after about 1 min, brackets left of curves) and at 25 min (right of curves) were located as indicated. *Left of symbol at 10 min in SHR indicates significant differences compared to the control group. *P < 0.0167 after curve analyses (please see Section “Materials and Methods” for details).

Figure 3

The HR response to 3,4-DAP after pre-treatment with reserpine or nadolol, alone or combined with atropine. Atropine + nadolol was given to SHR only. Significant responses (* within symbol) and group differences (* in brackets) at 1 (left of curves) and 25 min (right of curves) were located as indicated. Comparisons were made between the control and the experimental groups, and between the groups pre-treated with reserpine/atropine and reserpine + atropine, and between the nadolol- and atropine + nadolol-treated groups. *P < 0.025 after curve analyses.

Figure 4

The TPR response to 3,4-DAP after pre-treatment with cholinergic inhibitors or quinidine. The effect of quinidine was not tested in WKY. Significant responses (* within symbol) and group differences (* in brackets) at 1 (left of curves) and 25 min (right of curves) were located as indicated. *P < 0.025 after curve analyses.

Figure 5

The TPR and MBP response to 3,4-DAP after pre-treatment with reserpine or nadolol, alone or combined with atropine. The effect of nadolol + atropine was not tested in WKY. Significant responses (* within symbol) and group differences (* in brackets) at 1 (left of curves) and 25 min (right of curves) were located as indicated. Comparisons were made between the control and the experimental groups, and between the groups pre-treated with reserpine/atropine and reserpine + atropine, and between the nadolol- and atropine + nadolol-treated groups. *P < 0.025 after curve analyses.

Figure 6

The HR response to 4-AP compared to that following 3,4-DAP, and the effect of Ach synthesis inhibitor (hemicholinium-3) on the response to 4-AP. Arrows indicate the injection of 4-AP or 3.4-DAP. Significant responses (* within symbol) and group differences (* in brackets) at 1 (left of curves) and 25 min (right of curves) were located as indicated. *P < 0.025 after curve analyses.

Figure 7

The TPR and HR responses to nicotine, alone and after prior administration of 3,4-DAP. Arrows indicate the injection of nicotine. The immediate TPR response was recorded at the peak response, whereas the HR response was recorded at the nadir response, occurring a few seconds earlier. *P < 0.025 after curve analyses.