Proceedings of The Physiological Society
University College Dublin (2009) Proc Physiol Soc 15, PC148
ATP-mediated regulation of vasa recta diameter by pericytes in the renal medulla
C. Sprott1, R. L. Simmons1, C. Crawford1, T. Kennedy-Lydon1, R. J. Unwin2, S. S. Wildman1, C. M. Peppiatt-Wildman1
1. Urinary Systems Physiology Unit, Royal Veterinary College, London, United Kingdom. 2. Centre for Nephrology, University College London, London, United Kingdom.
ATP-activated P2 receptors are expressed throughout the kidney in vascular smooth muscle, endothelial and tubular epithelial cells. Functional studies demonstrate that the renal vasculature is highly responsive to P2 receptor activation, and in vivo studies in rabbits have shown that the ATP analogue α, β-methylene ATP reduces both cortical and medullary blood flow . In the renal medulla, blood flows via the vasa recta which are devoid of contractile smooth muscle. In vitro studies looking at isolated descending vasa recta have however shown that smooth muscle-like pericyte cells occur at regular intervals. Furthermore, it has been shown that these pericytes can constrict isolated descending vasa recta in the presence of certain vasoactive substances . In addition to studies on isolated capillaries, pericytes have been shown to control capillary diameter in situ, in whole retina and in the CNS . In the retina pericytes have been shown to respond to ATP [3, 4]. Here we aim to (i) investigate whether extracellular ATP and UTP constrict in situ vasa recta pericytes and (ii) identify expression of P2 receptor subtypes in the medulla. Kidney slices (200 μm) were obtained from Sprague Dawley rats (killed by cervical dislocation) and maintained in a physiological saline solution bubbled with 95% O2/ 5% CO2. Video imaging techniques were utilized to study whether ATP and UTP regulate vasa recta pericytes in live kidney slices; pericytes were identified by their 'bump on a log’ morphology . Immunohistochemistry techniques and confocal microscopy were employed to label and identify specific P2 receptor subtypes in fixed kidney slices. Superfusion of ATP (10 µM) evoked significantly greater vasoconstriction (p<0.05, Student’s t test) of vasa recta at pericyte sites (24.37±4.94%, mean±s.e.m, n=5.) than at non-pericyte sites (2.15±2.11%, mean±s.e.m.). 100 µM UTP also evoked significantly greater vasoconstriction (p<0.05, Student’s t test) of vasa recta at pericyte sites (8.63±1.38%, mean±s.e.m., n=12) than at non-pericyte sites (4.93±0.81%, mean±s.e.m.). Higher concentrations of UTP (1 mM) evoked a greater vasoconstriction of vasa recta at pericyte sites (15.48±1.84%, mean±s.e.m., n=5) but did not significantly constrict non-pericyte sites (5.17±0.84%, mean±s.e.m., p<0.05, Student’s t test). The P2 receptor antagonist suramin (100 µM) attenuated the ATP-evoked vasoconstriction by 56.12% (n=3). Immunohistochemistry experiments identified expression of P2X3, P2X4 and P2X5 receptor subtypes in the renal medulla. Data presented here demonstrate that P2 receptors are expressed throughout the medulla in rat kidney slices and that both ATP and UTP can act specifically at pericytes to regulate vasa recta diameter. We propose that pericytes play a key role in regulating medullary blood flow at the capillary level in the kidney.
Where applicable, experiments conform with Society ethical requirements