Relationship between mean arterial pressure and muscle cell phi during forearm ischaemia after sustained handgrip

Recent evidence suggests that there is a close correlation between the physiological responses to muscle chemoreflex and the decrease in intracellular pH during ischaemia after handgrip. This study evaluated whether the relationship is linear or has an apparent threshold. We measured muscle cellular pH through phosphorous nuclear magnetic resonance spectroscopy (31P-NMR), mean arterial blood pressure (MAP) and heart rate (HR) during ischaemia after sustained handgrip exercise at 50% of maximum voluntary contraction (MVC). Contraction was sustained for 15, 30, 45 and 60 s, followed by 2 min of circulatory arrest, respectively. Muscular pH during the ischaemia decreased linearly with increasing contraction time, from the base-line level of 7.11 +/- 0.03 units (U) to 6.98 +/- 0.03, 6.90 +/- 0.04, 6.72 +/- 0.06 and 6.54 +/- 0.06 U after 15-, 30, 45-, and 60-s contractions, respectively. The MAP was 86 +/- 2 mmHg at rest and did not change during the ischaemia after 15- and 30-s contractions. However, it significantly increased to 95 +/- 2 and 107 +/- 2 mmHg, after 45- and 60-s contractions, respectively. These data indicate that the relationship between MAP and pH is not a single linear relationship, showing one breaking point around the pH of 6.90 units. It suggests that the muscle chemoreflex has a clear threshold around 6.90 units of muscle pH, and below this pH, MAP increased linearly with decreasing muscle cellular pH.