Optimization of myocardial function.
Under normal conditions the cardiac output is designed to meet the metabolic needs of the organism. Thus, the demands imposed on the heart muscle can range from low values at rest to an order of magnitude greater values during exercise. The heart uses a number of strategies to meet the short- and long-term changes in demand. These strategies are of general biological interest and employ similar mechanisms to those responsible for the differences in muscle performance seen between muscle from various species and diverse muscle types within a given animal. This review deals with the heart's utilization of these strategies to meet a broad range of requirements. Tortoise (TM) and rat soleus (RS) muscles are slow, have high economy and develop low power. In contrast (FM) and rat extensor digitorum longus (REDL) are fast, have low economy and have a high power output. These differences are explainable in terms of the characteristics of the myosin head cross-bridge cycle (Cross-bridge tension-time integral: FM/FT = 0.024; REDL/RS = 0.16. Myosin ATPase activity: FM/TM = 15; RDEL/RS = 2.3) and excitation contraction coupling system (time to peak tension: FM/TM = 0.2; REDL/RS = 0.4). Heart muscle employs similar strategies (cross-bridge cycle; excitation contraction coupling) to meet short (catecholamine) and long (hypertrophy secondary to pressure overload or thyrotoxicosis) term changes in demand. In the presence of catecholamine power is increased while economy is decreased. This difference between control (C) and isoproterenol treated hearts (I) is explainable in terms of the contractile and excitation contraction coupling systems (Cross-bridge tension-time integral: I/C = 0.4. Tension independent heat: I/C = 2.0. Tension independent heat rate: I/C = 2.5). A persistent increase in the demand on the heart results in myocardial hypertrophy that is associated with intracellular reorganization. Hyperthyroidism (T) and pressure overload (PO) were used to produce myocardial hypertrophy. In T hearts the economy is decreased while the power is increased; in PO hearts oppositely directed changes occur. These alterations are attributable to changes in the performance of the contractile and excitation contraction coupling systems (Cross-bridge force-time integral: T/C/PO = 0.5/1.0/2.6. Tension independent heat: T/C/PO = 1.4/1.0/0.4. Tension independent heat rate: T/C/PO = 1.4/1.0/0.3). Thus it is clear that in meeting changes in demand, the heart uses strategies comparable to those seen between species and muscle types within a given muscle.
Alpert
N R
NR
University of Vermont, Department of Physiology and Biophysics, Burlington.
Mulieri
L A
LA
Hasenfuss
G
G
Holubarsch
C
C
D000818:Animals; D001833:Body Temperature Regulation / Q000502:physiology*; D006332:Cardiomegaly / Q000503:physiopathology; D006168:Guinea Pigs; D006801:Humans; D007545:Isoproterenol / Q000494:pharmacology; D009119:Muscle Contraction / Q000502:physiology*; D009200:Myocardial Contraction / Q000187:drug effects / Q000502:physiology*; D009218:Myosins / Q000378:metabolism*; D011817:Rabbits; D011898:Ranidae; D051381:Rats; D012104:Reptiles; D013997:Time Factors
D009218:Myosins; D007545:Isoproterenol