Does usefulness of potassium supplementation depend on speed?

REASONS FOR PERFORMING STUDY: Electrolyte mixtures given to counter sweat loss usually contain abundant potassium. However, increases in plasma [K+] occur with exercise and supplementation may further increase plasma levels, potentially increasing the risk of neuromuscular hyperexcitability and development of adverse clinical sequellae. This proposition requires study. OBJECTIVES: To compare effects of a K-rich electrolyte supplement (EM+K) to a K-free one (EM-K) on plasma [K+], [Ca++] and acid-base status during an endurance incremental exercise test on the treadmill. METHODS: The test consisted of 3 bouts (simulating loops in an endurance race) of 12 km performed at 6, then 7, then 8 m/sec with 25 min rest stops (S1, S2) between loops on 13 endurance trained Arabian horses (7 EM-K, 6 EM+K). Electrolytes were supplied orally 60 mins before exercise (Pre) and at each stop. Blood samples were taken before exercise and during exercise, each S and 120 mins of recovery (R). Blood was analysed for pH, PCO2, packed cell volume (PCV), plasma [Na+], [K+], [Cl-], [Ca++], glucose, and lactate [La-]; plasma [H+] and osmolality (osm) were calculated. The dietary cation anion difference (DCAD) was calculated to be -27 meq/dose EM-K and 109 meq in EM+K, respectively. RESULTS: Plasma [H+] decreased during the 6 and 7 m/sec loops, increased during the 8 m/sec loop, and returned to Pre at S1, S2 and R. Plasma [K+] was higher at 8 m/sec and plasma [Ca++] was overall lower in the EM+K group compared to EM-K. Other findings included higher overall PCV, overall glucose, and [La-] during the 8 m/sec loop (P<0.040) in EM+K compared to EM-K horses. CONCLUSIONS: EM+K supplementation leads to higher plasma [K+] increasing the risk of neuromuscular hyperexcitability during exercise. Acute effects of a lower DCAD in EM-K may have led to higher plasma [Ca++]. Potassium-rich electrolytes may have triggered the release of epinephrine, contributing to higher PCV, glucose release and increased lactate production. POTENTIAL RELEVANCE: Lower plasma [K+] and higher plasma [Ca++] with EM-K supplementation may help reduce the risk of conditions associated with neuromuscular hyperexcitability occurring especially during higher speeds in endurance races.

Seasonal vitamin A depletion in grazing horses is assessed better by the relative dose response test than by serum retinol concentration.

Vitamin A influences growth and reproduction in horses. A retinol dose response (RDR) test for retinol has been shown to be better than serum retinol concentration for assessing vitamin A status in other species, so we have compared these two methods in the horse. Forty-five Thoroughbred broodmares were assigned randomly to three groups fed pasture and hay (PH), pasture, hay and vitamin A-free concentrate (PHC), or hay and concentrate (HC) in early summer (May 1991). Mares in pasture groups produced 23 foals (March through June) that had access to their dam's diets and were also studied. In the mares, significant vitamin A depletion developed in 2 mo in the nonpasture group (HC) and in 8 mo in the two pasture groups (PH and PHC) according to the RDR test, and in all three groups at 8 mo as shown by a decrease in serum retinol concentration. In the weanlings (PH and PHC only), no differences between groups were found for serum retinol, but the RDR was significantly higher in the PH group, which had suffered a respiratory infection, than in the PHC group. These findings indicated that vitamin A depletion was detected more readily by the RDR test than by serum retinol concentration, that consumption of pasture delayed depletion in the late fall, and that infection was associated with lower vitamin A status.

The selectivity of the delayed potassium conductance of frog skeletal muscle fibers.

A voltage-clamp technique was used to measure reversal potentials of delayed currents in frog skeletal muscle fibres immersed in solutions containing Li2SO4,Na2SO4,K2so4,Rb2SO4, or Cs2SO4. The selectivity sequence found for the underlying permeability mechanism was K greater than or equal to Rb greater than Cs greater than Na greater than or equal to Li.

Cs(+) causes a voltage-dependent block of inward K currents in resting skeletal muscle fibres.

When frog skeletal muscle fibres are bathed in solutions containing Cs(+) and K(+) in the ratio 1:4,000, a reduction is observed in the size of inward K currents through the resting membrane. This effect is enhanced by an increase in either hyperpolarisation or external Cs(+) concentration. It can be predicted from these findings that regenerative changes in membrane potential should be obtainable in fibres, in the presence of Cs(+), that are hyperpolarised by means of a current electrode. Such responses are described in the last part of this report. In squid axon and frog node, internal Cs(+) produces a voltage-dependent block of the delayed, outward K currents, though the ratio of Cs(+) to K(+) required for this effect is far greater than that used in the experiments reported here. A closer parallel can be drawn between our findings and those recently reported on the inward K currents in the starfish egg cell.