Which of the following is traditionally used to evaluate the level of intensity of aerobic activity?

Physiology

Even at rest, muscle function requires a large amount of adenosine triphosphate (ATP). ATP generation by muscle accounts for 30% of the body's oxygen consumption at rest and up to 85% at extremes of physical activity. Resting muscle uses fatty acids for ATP generation. With activity, muscle draws on stored ATP for the first 8 seconds of activity, using the phosphagen (creatine phosphate) stores for the next 10 to 15 seconds. Finally, muscle depends on anaerobic glycogen metabolism to lactate for enough ATP for an additional 30 to 40 seconds of activity. Aerobic ATP production provides the bulk of the energy needed for muscle activity, but it requires oxygen. Glucose, amino acids, and fatty acids are incorporated into the Krebs cycle to produce much larger quantities of ATP by energy-rich compounds, such as the reduced forms of nicotinamide adenine dinucleotide and flavin adenine dinucleotide.

Myoglobin, like hemoglobin, binds and releases oxygen and delivers it to active skeletal muscle. Unlike hemoglobin, myoblobin's ability to deliver oxygen is unaffected by pH, resulting in a relatively increased affinity for oxygen in comparison to hemoglobin and delivery of oxygen to cellular mitochondria at low partial pressures of oxygen.

The integrity of muscle cells is dependent on healthy cell membranes, which rely on ATP for proper membrane ion pump function. The sarcolemma, a thin membrane that encloses striated muscle fibers, contains numerous pumps that regulate electrochemical gradients. Under normal physiologic conditions, the sodium-potassium–adenosine triphosphatase (Na+,K+-ATPase) pump, located in the sarcolemma, maintains intracellular sodium concentrations of 10 mEq/L and intracellular potassium concentrations of 150 to 160 mEq/L. It achieves this by actively transporting sodium from the interior of the cell to the exterior, thereby making the interior of the cell more negative by the efflux of net positive charge—three sodium ions pumped out per two potassium ions pumped in. This electrical gradient pulls sodium to the interior of the cell through a separate channel in exchange for calcium, effectively removing calcium from the cytoplasm. Low intracellular calcium levels are also maintained by an active calcium exchanger (Ca2+-ATPase pump) that promotes calcium entry into the sarcoplasmic reticulum and mitochondria. As their names indicate, these ATPase pumps depend on ATP as a source of energy.

Under normal physiologic conditions, the concentration of free ionized calcium in the extracellular space is approximately 10,000 times greater than that in the intracellular space. The high concentration of free calcium in the extracellular pool compared with the intracellular compartment and the resulting large electrochemical force on Ca2+ are particularly convenient to its role as an intracellular regulator. Even minor changes in the permeability of the plasma membrane to calcium will produce significant fluctuations in the cytosolic concentration, with potentially unfavorable consequences for the integrity of the cell.

1 PHOSPHAGEN UTILIZATION DURING EXERCISE

After about 2 minutes of exercise, the phosphagen concentration in quadriceps muscle has been shown to be reduced to a constant value relative to rest, with the change proportional to the work rate for bicycle ergometer exercise (14,92). Most of the phosphagen used is phosphocreatine, but there are some ATP stores also used during the early phase of exercise. As implied by curve 1 of Fig. 1, net consumption of phosphagen stores occurs immediately at the onset of exercise and is terminated once other sources of ATP production attain a level that satisfy the contemporary ATP requirement. Regardless of the intensity or duration of exercise, the net phosphagen stores used in the early part are not resynthesized later in the exercise (14,91,92). This finding suggests that when ATP production and utilization attain a new steady state during exercise, new equilibria for the creatine kinase [Eq. (3)] and adenylate kinase [Eq. (2)] reactions are attained. However, the study of this problem by Sahlin et al. (129) showed only the apparent equilibrium constant for creatine kinase (K’ck) increases; that for adenylate kinase was not changed. Because of cellular compartmentalization and the difficulty of identifying the free species of the reactants, it is impossible to explicitly evaluate the mechanism for the increase in K’ck. Of the several possibilities, the alteration of intracellular pH and change in that portion of ADP and ATP which is available to creatine kinase enzyme likely are most important for affecting K’ck. Sahlin et al. (129) found the changes in intracellular pH of muscle is highly related (r = 0.92, n = 34) to K‘ck, that is, the theoretical expression for the equilibrium constant of creatine kinase, when transformed to solve for pH, is

(5)pH=−log[Cr][ATP][PC][ADP]+log KpH=−log(Kck′)+ log K

and the relationship actually found was

(6)pH = −0.42  log[Kck′] + 7.38

This finding and the fact that K’ck for adenylate kinase was not observed to change suggested to them the new equilibrium established for creatine kinase during exercise is determined mainly by the change in intracellular pH. That is, H+ ion activity increases in muscle soon after the onset of exercise, due primarily to lactate production (80), and helps to drive the creatine kinase reaction [Eq. (3)] to the right. The result is a new steady state, achieved with muscle PC maintained at a lower level throughout the remainder of exercise. The net amount of PC used during exercise is proportional to the work rate (14). PC is resynthesized back to its preexercise level only during the recovery period.

Phosphocreatine-Creatine System

Phosphocreatine (also calledcreatine phosphate) is another chemical compound that has a high-energy phosphate bond, with the following formula:

Creatine∼PO3−

Phosphocreatine can decompose tocreatine andphosphate ion, as shown inFigure 85-2, and in doing so releases large amounts of energy. In fact, the high-energy phosphate bond of phosphocreatine has more energy than the bond of ATP: 10,300 calories per mole compared with 7300 for the ATP bond. Therefore, phosphocreatine can easily provide enough energy to reconstitute the high-energy bond of ATP. Furthermore, most muscle cells have two to four times as much phosphocreatine as ATP.

A special characteristic of energy transfer from phosphocreatine to ATP is that it occurs within a small fraction of a second. Therefore, all the energy stored in muscle phosphocreatine is almost instantaneously available for muscle contraction, just as is the energy stored in ATP.

The combined amounts of cell ATP and cell phosphocreatine are called thephosphagen energy system. These substances together can provide maximal muscle power for 8 to 10 seconds, almost enough for the 100-meter run.Thus, the energy from the phosphagen system is used for maximal short bursts of muscle power.

SOURCES OF ENERGY AND CATABOLIC PROCESSES

Sources of energy for muscular contraction include the phosphagens and the aerobic and anaerobic catabolism of energy-reserves. The extent to which each of these occurs and is used in a muscle tissue depends upon the tissue's particular nature and function. Muscles which contract slowly and regularly over long periods of time tend to employ aerobic catabolism (oxidative phosphorylation) whereas those which contract vigorously during short bursts of mechanical activity employ anaerobic glycolysis. Mitochondria and blood supply are important to the former and high levels of phosphagen and glycolytic enzymes to the latter. A third situation exists in the Mollusca in that muscles can be deprived of oxygen for hours, days or weeks for environmental or physiological reasons, for example during tidal excursions or hibernation. The Bivalvia are also unique in possessing the so-called catch mechanism (Twarog, 1976) to maintain valve closure. These special smooth muscle fibres can remain contracted for long periods of time with minimal energy expenditure. They may occur mixed in with fast contracting striated fibres, as in the adductor muscle of Mytilus edulis, or they may form a catch muscle distinct from a phasic (striated) muscle, as in scallops.

Phosphoarginine is the high-energy phosphorylated compound (phosphagen) of the Mollusca which is broken down by the action of arginine phosphokinase to yield ATP (phosphoarginine + ADP = arginine + ATP). Concentrations vary with the type of muscle (Table 1). They are highest in the rapidly contracting phasic muscle of swimming bivalves, mantle muscle of cephalopods and foot of the jumping cockle C. tuberculatum (overall mean concentration calculated from Table 1 of 23.1 ± 2.8 (S.E.) μmoles g−1 wet weight; n = 9), lower in slowly contracting muscles of sessile bivalves and catch muscles of swimming bivalves (5.4 ± 1.2; n = 5) and very low in the essentially aerobic tissue of the fileshell L. fragilis. Activities of arginine phosphokinase are similarly higher in the fast contracting muscles (De Zwaan, 1977; De Zwaan, Thompson and Livingstone, 1980). Rates of phosphoarginine utilization vary with energy demand, being high during mechanical activity (squid mantle: 39.9 ± 8.6 (n = 3) μ moles g−1 min−1 and swimming-bivalve adductor muscle: 5.0 ± 1.0; n = 6) and low during anaerobiosis (sessile bivalve muscle: 0.014 ± 0.005; n = 4) (data calculated from Table 1). The differences in rate reflect the near-equilibrium nature of the reaction (Beis and Newsholme, 1975) which is well-illustrated where a tissue is given different stimuli e.g. the foot of C. tuberculatum and the catch muscle of P. magellanicus (Table 1). Glycogen is the main catabolic energy-source, at least anaerobically (Goddard and Martin, 1966; De Zwaan, 1977), while hexokinase activities indicate free glucose is important for some tissues viz. sessile bivalve phasic muscle, prosobranch radular muscle and squid fin muscle (Zammit and Newsholme, 1976). Rates of glycogen utilization are lowest where tissue concentrations are highest (Table 2), reflecting that some of these muscles also function as storage tissues. Glycogen levels are low in many of the muscles of cephalopods (Table 2; also Goddard and Martin, 1966; Suryanarayanan and Alexander, 1971) suggesting that other energy-sources such as lipid may be involved in aerobic catabolism; proline has been suggested as an aerobic substrate in squid mantle (Hochachka and co-workers, 1975; Storey and Storey, 1978). With the exception of aspartate, there is no evidence that amino acids are a general energy-source for anaerobic muscle contraction (De Zwaan, 1977).

TABLE 1. Distribution of phospho-L-arginine (PARG - μmoles g−1 wet weight) and rates of utilization (RU - μmoles g−1 wet weight min−1) in muscle tissues of different Mollusca.

Molluscan-typeSpeciesTissuePARGRUActivityReferenceBIVALVIA (free-swimming)Limaria fragilisa.1.20.3–0.1Sustained swimmingBaldwin and Lee (1979)Lima hiansa.22.66.8Escape swimming(1981)Pecten jacobaeusa.12.61.4Escape swimmingGrieshaber and(1977)Pecten maximusa.25.84.5*Escape swimmingGade, Weeda and Gabbott (1978)Chlamys opercularisa.20.43.8*Escape swimmingGrieshaber (1978)Pecten albapa.24.94.5Escape swimmingBaldwin and Opie (1978)ca.3.8No changeEscape swimmingPlacopecten magellanicuspa.22.38.8Escape swimmingDe Zwaan, Thompson and Livingstone (1980)ca.5.11.5Escape swimmingca.5.10.03Valve closureBIVALVIA (sessile)Mytilus edulispam.8.5–2.0 †0.011Aerial exposureZurburg and Ebberink (1981)Cardium edulef.7.40.017AnoxiaMeinardus and(1981)f.7.40.6Electrical stimulationCardium tuberculatumf.240.019Anoxia(1980)246.8JumpingCEPHALOPODALoligo pealeiim.10.55.7Burst swimmingStorey and Storey (1978)Loligo vulgarism.35.6‡32.8**Burst swimmingGrieshaber and(1976)Sepia officinalism.33.629.9**Burst swimmingStorey and Storey (1979)GASTROPODABuccinum undatumcm.8.80.24Escape movementsKoormann and Grieshaber (1980)Helix pomatiaf.1.4––Beis and Newsholme (1975)

a: adductor muscle; pa: phasic adductor muscle; ca: catch adductor muscle; pam: posterior adductor muscle; f: foot; m: mantle muscle; cm: columnellar muscle. Rates calculated from the Reference using decrease in phosphoarginine per unit time

TABLE 2. Glycogen levels (mgs g−1 dry weight) and rates of utilization (RU - mgs g−1 dry weight hr−1 (A) or μmoles glucosyl units g−1 wet weight min−1 (B) in muscle tissues of different Mollusca.

Molluscan-typeSpeciesTissueGlycogenRUActivityReferenceA.B.BIVALVIA (free–swimming)Placopecten magellanicuspa.310No changedetectableEscape swimmingDe Zwaan,ca.126No changedetectableEscape swimmingThompson, andca.126No changedetectableValve closureLivingstone (1980)BIVALVIA (sessile)Mytilus edulispam.75–200No changedetectableAerial exposureDe Zwaan (1977); Zandee and coworkers (1980)Cardium edulea.391.70.028Anoxia(1975)Unio sp.a.1171.50.025Anoxia(1975)CEPHALOPODASepia officinalism.0.9511.52*19.2*Burst swimmingStorey and Storey (1979)Symplectoteuthis oualaniensism.15–––Hochachka and co-workers (1975)GASTROPODALymnaea luteolaf.67–––Manohar and Rao (1976)

Muscle abbreviations as for Table 1. Glycogen levels and rates of utilization calculated from the Reference using a water content figure of 80 per cent, molecular weight of 180 and a glycogen to glucosyl unit conversion figure of 1.111 (Morris, 1948).

The presence of Krebs cycle enzymes (Alp, Newsholme and Zammit, 1976), cytochromes and mitochondria (Zaba, De Bont and De Zwaan, 1978) in muscle tissues indicate that their basal metabolism is probably aerobic when oxygen is available. Examples of aerobic catabolism supporting active muscle contraction, however, are few to date. Enzyme studies indicate that squid fin muscle and the prosobranch radular muscle which tend to be mechanically active for long periods of time operate aerobically (Zammit and Newsholme, 1976); the latter muscles have been likened in biochemical content to vertebrate red muscle (Suryanarayanan and Alexander, 1973). Aerobic catabolism also maintains the slow sustained swimming of the fileshell Limaria fragilis (no anaerobic end-products accumulated but oxygen consumption increased 8-fold) (Baldwin and Lee, 1979) and the cruiseswimming of the cephalopod Symplectoteuthis oualaniensis (Hochachka and co-workers, 1975). In the latter case, high activities of mantle ∝-glycerophosphate dehydrogenase and other considerations indicate that a situation similar to insect flight muscle may exist with cytoplasmic reducing equivalents being rapidly transferred into the mitochondria by an active ∝-glycerophosphate cycle. A vigorous aerobic metabolism for certain areas of squid mantle is also supported by the observation of well-vascularised mitochondria-rich fibres in these tissues (Bone, Pulsford and Chubb, 1981).

In contrast to aerobic catabolism, examples of anaerobic muscle contraction are many and are described in the rest of the paper. An important consideration in this catabolism is the nature of the anaerobic pathway employed to utilise the energy-substrate. In the Mollusca, three types of pathway can be identified which differ in the amount of ATP that they produce per glucosyl unit and/or the rate at which ATP is produced. The pathways are termed the lactate, opine and succinate pathways and their different energetic characteristics render them useful for different functions in the muscles of the Mollusca. Whereas the former two tend to be employed where increased rates of energy production are required, the latter is concerned with survival under anoxic conditions (anoxia-survival).

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