Acetylcholine acts as an agonist of both muscarinic and nicotinic receptors. However, the affinity for muscarinic receptors is much greater than the affinity for nicotinic receptors. Therefore, unless the muscarinic receptors are blocked one almost never observes nicotinically mediated responses following injection of acetylcholine. The muscarinic receptors that promote vasodilation are located on endothelial cells. The receptors are not innervated, but upon encountering an agonist they cause the endothelial cells to secrete nitric oxide (endothelial derived relaxation factor, EDRF). Nitric oxide diffuses to the vascular smooth muscle cells and activates the soluble guanylate cyclase which, through a series of steps, causes vascular relaxation. The very short duration of the response is due to the rapid hydrolysis of acetylcholine by acetylcholinesterase. What is an example of a drug that would potentiate the response to injected acetylcholine?
Carbachol acts on both muscarinic and nicotinic receptors. As in the case of acetylcholine, the muscarinic effects usually predominate unless the muscarinic receptors are blocked. The longer duration of action of carbachol as compared to acetylcholine is due to its lack of hydrolysis by acetylcholinesterase. This longer duration of action also helps one to demonstrate the nicotinic effects of Carbachol (when the muscarinic responses are blocked).
Carotid occlusion means that both carotid arteries are pinched off below the bifurcation; for about 10 seconds in the example shown here. Thus, the carotid baroreceptors suddenly sense a profound drop in arterial pressure. This results in reflex activation of the sympathetic nervous system. Generalized sympathetic discharge occurs, prompting the release of norepinephrine from postganglionic nerve terminals. This causes positive chronotropic (not seen on this time base), positive inotropic (increased pulse pressure) and positive bathmotropic (increased AV nodal conduction velocity, which could be seen by EKG) and increased total peripheral resistance. Of course, there is also activation of adrenal medullary release of catecholamines (including a substantial fraction of which is epinephrine) as well.
DMPP, although not used clinically, is a useful example of a drug which is a relatively selective nicotinic receptor agonist. In the setting of the anesthetized dog, DMPP causes relatively selective activation of the release of catecholamines from the adrenal medulla. There is generalized activation of autonomic ganglia, but usually the adrenal response predominates.
Epinephrine acts on alpha receptors causing vasoconstriction and on beta receptors causing vasodilation. The affinity of epinephrine for beta receptors is somewhat greater than its affinity for alpha receptors. When given in low doses, or by slow IV infusion in humans, the beta effects of epinephrine may predominate. When given in a large IV bolus dose, as here, the alpha effects (vasoconstriction) predominate when the concentration of epinephrine is high, and the beta effects (vasodilation) may sometimes be seen as the concentration is falling. From these facts you may be able to infer the relative affinity of epinephrine for alpha receptors on the one hand, and beta receptors on the other hand.The fact that vasoconstriction predominates when both alpha and beta receptors are activated shows that the capacity for vasoconstriction mediated by alpha receptors is very great, whereas the capacity for vasodilation mediated by beta receptors is somewhat limited.
Histamine induced vasodilation is mediated both by H1 and H2 receptors which are both widely distributed in resistance vessels. H1 receptors mediate a rapid onset, short lived vasodilation. H2 receptors mediate a slower onset, longer duration vasodilation. H1 receptors are located on endothelial cells and promote the release of local vasodilator substance(s).
Isoproterenol is the classical beta-receptor agonist. The depressor response observed here is due to activation beta-2 receptors. These are located mainly in vascular beds of skeletal muscle. The increased pulse pressure that is apparent in this response is a reflection of the activation is isoproterenol of the beta-1 receptors in the heart muscle. Because of the time base of the display, the increased heart rate also caused by isoproterenol is not apparent. The example here illustrates a phenomenon often seen when isoproterenol is injected rapidly intravenously in an anesthetized dog. Thus, there is a very brief pressor response prior to the main depressor response. Try to come up with an explanation of this brief pressor response.
McN-A-343 is a unique muscarinic agonist with selectivity for the M1 muscarinic receptors located on nerves. In this experiment, the major pressor response elicited by McN-A-343 is caused by activation sympathetic ganglia via M1 receptors with subsequent postganglionic transmission to nerve endings with the release of norepinephrine onto innervated structures. In this case we observe a pressor response caused by vasoconstriction elicited by the release of NE from sympathetic nerve endings. The small depressor response seen before the pressor response is probably a reflection of minor activation of M3 receptors on the endothelium. Thus, the response illustrates that, while it is selective for M1 receptors, the selectivity shown by McN-A-343 is not absolute. This is typical of most supposedly selective agents.
Norepinephrine causes increased peripheral vascular resistance and increased blood pressure by acting on alpha1 receptors. NE also acts on beta1 receptors in the heart where it tends to increase heart rate (not noticeable on this time scale). Remember that the positive chronotropic response to NE is generally overwhelmed by the vagal reflex in a non-vagotomized or otherwise untreated animal. In contrast to EPI, NE does not act on beta2 receptors in blood vessels.
Phenylephrine is a selective alpha agonist. Thus it has vascular properties similar to norepinephrine but without the cardiac stimulation. Also, it has no propensity to cause vasodilation like epinephrine. It has a longer duration of action than the catecholaminess because it is not actively transported into sympathetic nerve terminals by the amine uptake pump.
Tyramine is a indirectly acting sympathomimetic amine. It is take up into sympathetic nerve terminals, and releases NE from a small tyramine releasable pool. This released NE produces the sympathomimetic effect. Uptake of tyramine into sympathetic nerves is blocked by drugs such as cocaine or impramine. The tyramine releasable pool, as the action potential releasable pool of NE in sympathetic nerve terminals is depleted by reserpine. The release of NE from both pools is inhibited by bretylium. Tyramine does not cause much release of catecholamines from the adrenal medulla because it is not taken up there. This is probably because the adrenal medulla is in the business of releasing hormone catecholamines, whereas nerve terminals are in the business of releasing (and re-releasing) neurotransmitter catecholamines.
The depressor response caused by vagal stimulation is different from all others seen in this experiment. Almost all drugs or procedures used in this experiment alter blood pressure by changing total peripheral vascular resistance (and, to some extent, cardiac output). Stimulation of the distal stump of the cut vagus nerve (stimulating the part going to the heart) results in profound slowing or stoppage of the heart. It does not cause vasodilation because there is little or no parasympathetic innervation of vascular smooth muscle (compare this response to that of injected acetylcholine). Eventually, the heart will resume beating, even if the stimulation continued. This is so-called vagal escape, which is mediated mainly by reflex activation of the sympathetic nervous system. This sympathetic activation is responsible for the increased blood pressure and pulse pressure after the vagal stimulation is discontinued.
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