RESPIRATORY CONTROL DURING EXERCISE - HORMONES, OSMOLALITY, STRONG IONS, AND PACO(2)

For optimal performance of exercising muscle, the charge state of prot eins must be maintained; the pH environment of protein histidine imida zole groups must be coordinated with their pK. During exercise, increa sing temperature and osmolality as well as changes in strong ions affe ct the pK of imidazole groups. Production of strong organic anions als o den eases the concentration difference between strong cations and an ions (strong ion difference, or [SID]), causing a metabolic acidosis i n peripheral tissues. Central chemoreceptors regulate Peg in relation to the [SID] of brain fluids to maintain a ''constant'' brain [H+]. In addition, increased osmolality, angiotensin II, and vasopressin durin g exercise may stimulate circumventricular organs of the brain and int eract with chemical control of ventilation. Changes in [SID] of brain fluids during exercise are negligible compared to systemic decreases i n [SID]; thus, regulation of Pco(2) to maintain brain [H+] homeostasis cannot simultaneously compensate for greater changes in [SID] in peri pheral tissues.