Diurnal changes in ouabain-sensitive Na+,K+-ATPase activity in the rat spinal dorsal horn

Abstract

Ouabain-sensitive Na+,K+-ATPase (NKA) catalyses active Na+/K+ exchange, and it is responsible for the Na+/K+ gradient across the plasma membrane, cell volume control and membrane excitability and it has been associated to nociception in the spinal dorsal horn (SDH). Several tissues show diurnal changes in NKA activity with important physiological consequences. To test the possibility of diurnal changes in NKA activity in the SDH, every 2 hours, 2 adult (~300 g) male Sprague-Dawley rats housed at a 12: 12 h light: darkness schedule were anesthetized (thiobarbital) and laminectomized (T11–L5), to extract the lumbar spinal cord. Micropunches of SDH (1 mm o) were taken from coronal sections of the spinal cord, and their homogenates were tested for protein concentration and NKA activity. The NKA activity exhibited a highly significant rhythm with a 21.2 h period (P < 0.005; robustness = 57.9%, diurnal fit P < 0.02), a peak at 15.93 h and a 24 h mean value of 638.3 nmol P i mg prot–1 min–1, with an amplitude of oscillation of 71.0 nmol P i mg prot–1 min–1. Light hours show significantly higher ATPase activity values than dark hours (660.9 ± 51.0 vs 608.4 ± 12.8 nmol P i mg prot–1 min–1; P = 0.03). Thus, ouabain-sensitive NKA in SDH displays an intense diurnal activity that could be an important factor in pain and sensory diurnal changes.