Effects of β-hydroxybutyrate and different calcium and potassium concentrations on the membrane potential and motility of abomasal smooth muscle cells in cattle.

The left displacement of the abomasum (LDA) is a common disease in periparturient dairy cows. Plasma β-hydroxybutyrate (BHBA) levels above the reference range are regarded as risk factors for the occurrence of LDA. Additionally, hypokalemia and hypocalcemia have been observed in LDA cows. The aim of the present study was to characterize the membrane potential and the slow waves in abomasal smooth muscle cells by a microelectrode technique and to determine possible effects of BHBA and of various calcium and potassium concentrations on the membrane potential. Subsequently, the results obtained by the microelectrode technique were combined with in vitro motility experiments of abomasal smooth muscles. Strips of the abomasal circular smooth muscles were prepared and incubated in different buffer solutions. For the microelectrode technique, healthy bulls and cows that underwent surgery for LDA were sampled. These measurements showed a frequency of the slow waves between 3.5 and 10.9 per minute (for amplitudes ≥ 3mV) and between 0.6 and 4.5 per minute (for amplitudes ≥ 5mV). The frequency of contractions (1.8 to 3.1 per minute) were in the same order as the frequency of the slow waves with amplitudes ≥ 5 mV. Blocking potassium conductance with barium chloride induced a depolarization of the basal membrane potential (from -43±2.9 to -37±4.1mV; mean ± standard error of the mean) without affecting the frequency or the height of the slow waves. The reduction in the potassium concentration from 5.4 to 2 mmol/L resulted in a nominal decrease in the activity of contractions (from 22.2 to 18.6 mN/min). The subsequent addition of 1 mmol of KCl/L induced a nominal increase in contraction activity (from 18.6 to 25.7 mN/min). An effect of BHBA (5 mmol/L) could not be demonstrated, neither on the electric nor on the motility parameters. A simulated hypocalcemia (1.2 mmol/L total, 0.9 mmol/L ionized Ca) did not change slow waves and motility. In conclusion, changes in membrane potential ≥ 5 mV correlated with contractions of abomasal muscles. Hypokalemic conditions may reduce abomasal contraction activity via an effect on the membrane potential. An exclusive increase in BHBA or a slight hypocalcemia (0.9 mmol/L ionized Ca) had no effects on the motility of healthy abomasal smooth muscle cells. A prolonged or severe exposition of muscles to a combination of low Ca and high BHBA concentrations might nevertheless be able to affect abomasal motility.