Mediators of tubuloglomerular feedback regulation of glomerular filtration: ATP and adenosine

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

P2X receptors as regulators of the renal microvasculature

P2 receptors are expressed by renal vascular, glomerular, mesangial and tubular epithelial cells, suggesting that extracellular ATP serves a diverse array of physiological roles in regulating renal hemodynamic and tubular function. Evidence indicates that ATP, or its analogues, alter renal vascular resistance and renal blood flow significantly in vitro and in vivo. This review will focus on the recent evidence that supports extracellular ATP as an important regulator of renal microvascular function. The vascular actions of ATP involve the activation of P2X receptors to regulate renal vascular function, renal blood flow and also to mediate renal autoregulatory behavior. The review will also consider the implication that renal microvascular dysfunction in disease is related to P2 receptor dysfunction and we highlight some important issues and challenges that require further attention.