... the load on the myocardial muscles just prior to the onset of contraction
Preload influence contractility due to:
Pulmonary capillary wedge pressure (PCWP) (or pulmonary artery occlusion pressure (PAOP)) correlates well with LAP most of the time
... the impedance to the ejection of blood from the heart into the arterial circulation
[KB2:p47] ... the factor that is responsible for changes in myocardial performance which are not due to changes in heart rate, preload, or afterload.
[BL8] Performance of the heart at a given preload and afterload.
All mechanisms that increase contractility do so by increasing intracellular [Ca2+]
e.g. decrease in sodium gradient, increase in extracellular [Ca2+]
NB:
e.g.
Sympathetic stimulation
--> beta1 receptor stimulation
--> Increased cAMP
--> Increased contractility and faster relaxation
Also, catecholamines also increase sensitivity of myofilaments to Ca2+
(See ANS innervation of the heart)
e.g.
Parasympathetic stimulation
--> Muscarinic acetylcholine
--> Decreased cAMP
--> Decreased Ca2+ influx
--> Decreased contractility
(See ANS innervation of the heart)
Mainly epinephrine, and some norepinephrine
i.e. corticosteroids
Possibly potentiating in human [BL8:p109]
But is permissive for the permissive effect of catecholamine [WG21: p372]
Enhance contractility via
Prominent, direct, positive inotropic effect [BL8:p111]
Potent positive inotropic and chronotropic effect on the heart
Mechanism very similar to catecholamines.
* i.e. also increases cAMP via adenylyl cyclase
NB:
Inhibits Na-K ATPase pump
--> Decrease Na+ chemical gradient
--> Decreased Na+ influx
--> Decrease Ca2+ efflux via Na/Ca exchanger
--> Increase in intracellular Ca2+ level
--> Increase contractility
Also increases inward Ca2+ current
--> Also increase intracellular Ca2+ level
--> Increase contractility
(e.g. caffeine, theophylline)
Inhibit breakdown of cAMP
--> Inotropic effect
About | |
Created | 20050607 |
Reviewed | 20050815 |