ABSTRACT
The actions of the respiratory muscles are required for life. Like the heart, but in contrast with typical skeletal muscles, they must be continuously active in supporting pulmonary ventilation and must adapt to long-term alteration in respiratory loads. The performance of any skeletal muscle under such conditions is dependent on three main factors: (I) adequate muscle blood flow and oxygen delivery, (2) sufficient mitochondrial content, and (3) economical ATP turnover, determined by the presence of specific myosin and sarcoplasmic reticulum ATPase isoforms (see Holloszy, 1988; Laughlin and Armstrong, 1985; Rail, 1985; Terjung and Hood, 1986, for reviews). The important cellular factors responsible for ATP provision (glycolytic and mitochondrial metabolism) and utilization (calcium cycling and cross-bridge cycling) along with blood flow and 0 2 delivery are illustrated in Figure I. Fortunately, these are not static, immutable components of the muscle cell. Rather, they illustrate a "plasticity" or adaptability, that permits the cell to adjust to changing energy demands over the long term. This occurs in response to regularly performed muscular exercise, usually in the form of endurance training, but also, more dramatically, as induced experimentally with the use of continuous, indirect nerve stimulation. For the respiratory muscles, modifications of blood flow, mitochondrial content, and contractile protein isoforms can also potentially occur during the adaptation to prolonged pathological respiratory load (e.g., emphysema), or in response to specific respiratory muscle training. The adaptations that occur
314 lanuzzo and Hood
Figure 1 Performance of any skeletal muscle is dependent on the delivery of oxygen, the utilization of oxygen within the mitochondria for the synthesis of A TP, and the utilization of that A TP in the processes of Ca 2+ handling and actin-myosin interaction for the purposes of force generation and relaxation. As reviewed in the text, adaptations of these factors in response to physiological demand are important for the maintenance of muscle performance and the avoidance of fatigue.
