adrenoreceptor 3 Flashcards
what key physiological response is adrenoreceptors linked to ?
A key physiological response that adrenoceptors are linked to is the fear, flight of fight or response which is part of the sympathomimetic system.
explain effect on heart ?
An increase in heart rate and force of contraction of the heart muscle. Clearly you need to send more blood quicker to the organs that need it to run away!
effect on glycogen ?
An increase in the breakdown of glycogen to glucose in the liver. This means blood sugar levels are increased and can provide a source of energy in different organs such as the brain and skeletal muscle.
blood diversion ?
Blood is diverted to skeletal muscle to aid faster movement and heart so you can keep the increased activity going (the heart has its own supply via the carotid arteries) and the brain- you need to be able to react and think.
dilation ?
Increased blood via dilatation of blood vessels near or within these tissues. Clearly you need more blood to these organs/tissues
side effect of FFF?
A side effect is you have a dry mouth and clammy skin- for example at this point the body is not really interested in digestion! - so there is a negative effect on these systems.
Also, part of the Flight, fight or fear response is sweating as this cools the body down, bronchodilation occurs to increase air and oxygen into the lungs as you’ll be breathing quicker. The pupils dilate as you need to see clearly and sharply.
what is alpha 1 adrenoreceptor mainly cause ?
The alpha 1 adrenoreceptor causes constriction in several tissues. Blood vessels - contraction by alpha 1 receptor is mediated through Gq and Ca2+
Bronchi are contracted in response to alpha 1 receptor- this might seem a little strange but in fact airway vessels need a little bit of tone- you can’t have a floppy airway – no air would get through it!
Bladder - contraction is required to empty, so this seems sensible (but remember the overall effect of the sympathomimetic system is to inhibit urination due to a beta action).
Prostate and vas deferens - contraction is required for ejaculation
Eye - you have smooth muscle which lies in a radial configuration. Following contraction, the pupils dilate, and more light can enter the eye.
GI tract you have relaxation
why vasoconstriction of blood vessels?
Vasoconstriction of the blood vessels inhibits blood flow to the skin, kidneys, gut and liver – areas where during FFF you would not prioritise. Interestingly there is far less alpha 1 mediated effects on blood flow to the heart, lungs, brain and skeletal muscle. You don’t want to stop blood flow to these tissues during FFF.By blocking blood flow to one area it can be directed to another. It also suggests that blood vessels in heart, lungs, brain and skeletal muscle have fewer alpha 1 adrenoceptor (so no contraction).
when is noradrenaline released alpha one ?
uring sympathomimetic activity Noradrenaline is released locally from the nerve and alpha 1 adrenoceptors are close to the NA release site and are activated to cause contraction. However, NA is also rapidly destroyed at the site. Alpha 1 activation can however also be activated further by circulating NA which might for example come from the adrenal medulla which can act on alpha 2 receptors.
noradrenaline alpha 2 ?
For alpha 2 receptors the situation is different. They are also on the smooth muscle cell but far away from the site of NA release and more often only activated by circulating NA. Thus, they might be activated only in more severe conditions of FFF when lots of NA is circulating in the blood and the strength of contraction would reflect this.So this is a fascinating system in which two receptors can activate in concert, based on the source of NA and different locations of the receptors.
in neurones why is activation of alpha 2 important ?
Neurons the activation of alpha 2 has some important effects;
1) Hyperpolarisation (by an effect on K+ channels)
2) Inhibition of Ca2+ channels and as a result.
3) decrease in transmitter release
All these outcomes dampen down neuronal activation.
in neurones why is brain alpha 2 receptors activated ?
In neurons in the brain Alpha 2 receptors are activated and via Gi/Go this regulates potassium channels and allows K+ to leave the cells. This causes an effect called hyperpolarisation. This causes a reduction in nerve activity in 2 ways. Firstly, it causes a reduction in response to any agents which are excitatory i.e. causes the cell to depolarise. Many neurons have spontaneous activity as they depolarise once they reach a threshold again activation of alpha 2 receptor reduces this.
action potential of neurone ?
Action potential of a neuron in the brain, remember nerve cells create an action potential when they reach a certain point – called the threshold. That can be achieved by excitatory agents as in the top panel or it can happen spontaneously as in the bottom. Cells with spontaneous action potentials have a stream of firing.
effect of hyperpolarisation of spontaneuos firing ?
If you look at the bottom panel hyperpolarisation reduces the spontaneous firing. It increases the time delay when the cell itself can reach the threshold to activate an action potential and thus the spontaneous action potentials are less frequent
inhibitory effect of alpha 2 ?
The inhibitory effect of alpha 2 activation on nerve activity is important within the brain in relation to activation of the sympathetic nervous system. In the part of the brain called the Nucleus tractus solitarius (NTS) deals with basic functions, activation of alpha 2 receptors acting post synapticlly reduces the descending electrical impulses which travel outwards from the spinal cord and to the tissues. Thus at these tissues such as the blood vessels and heart the release of nor adrenaline is reduced, therefore activation of alpha 2 receptors in the brain can lower blood pressure.Alpha 2 agonists have been examined for their potential as anti-hypertensive agents (i.e. blood pressure lowering medicines).
