Contribution of pH, diprotonated phosphate and potassium for the reflex increase in blood pressure during handgrip

The relative importance of pH, diprotonated phosphate (H₂PO₄^-) and potassium (K⁺) for the reflex increase in mean arterial pressure (MAP) during exercise was evaluated in seven subjects during rhythmic handgrip at 15 and 30% maximal voluntary contraction (MVC), followed by post-exercise muscle ischaemia (PEMI). During 15% MVC, MAP rose from 92 ± 1 to 103 ± 2 mmHg, [K⁺] from 4.1 ± 0.1 to 5.1 ± 0.1 mmol L^-1, while the intracellular (7.00 ± 0.01 to 6.80 ± 0.06) and venous pH fell (7.39 ± 0.01 to 7.30 ± 0.01) (P < 0.05). The intracellular [H₂PO₄^-] increased 8.4 ± 2 mmol kg^-1 and the venous [H₂PO₄^-] from 0.14 ± 0.01 to 0.16 ± 0.01 mmol L^-1 (P < 0.05). During PEMI, MAP remained elevated along with the intracellular [H₂PO₄^-] as well as a low intracellular and venous pH. However, venous [K⁺] and [H₂PO₄^-] returned to the level at rest. During 30% MVC handgrip, MAP rose to 130 ± 3 mmHg, [K⁺] to 5.8 ± 0.2 mmol L^-1, the intracellular and extracellular [H₂PO₄^-] by 20 ± 5 mmol kg^-1 and to 0.20 ± 0.02 mmol L^-1, respectively, while the intracellular (6.33 ± 0.06) and venous pH fell (7.23 ± 0.02) (P < 0.05). During post-exercise muscle ischaemia all variables remained close to the exercise levels. Analysis of each variable as a predictor of blood pressure indicated that only the intracellular pH and diprotonated phosphate were linked to the reflex elevation of blood pressure during handgrip.