Expression and regulation of water channels and ion transporters : ontogenic aspects

Abstract

The content and distribution of body water changes dramatically during the perinatal period and in infancy. Transport of water across the cell membrane occurs via diffusion and via specific water channels. Transport via water channels is fast and may be hormonally regulated.

The main purpose of this thesis has been to study the peri- and postnatal development and regulation of water channels. Since water movement via water channels is dependent on ionic gradients, I also studied the postnatal expression of some ion transporters. Removal of rat lung liquid at birth was associated with pronounced changes in the expression of water channels, aquaporins (AQP) and ion transporters. The level mRNA levels of AQP1, Na+,K+-ATPase al, and amiloride sensitive sodium channel increased from embryonic day 18 (E18) to birth and continued to increase postnatally. There was a sharp and transient induction of AQP4 mRNA. The AQP4 protein was present in bronchial epithelial cells, where postnatal lung liquid clearance mainly takes place. Exposure of fetal rats (E20) to B-adrenergic agonists or glucocorticoids (GC) significantly increased lung AQP4 mRNA. These observations suggest that AQP4 plays a critical role for absorption of lung fluid at birth. The capacity to concentrate urine develops postnatally and is accelerated in the weaning period in rats. A similar development pattern was found for the AQP2 mRNA expression.

The expression of the mRNA of other transporters responsible for medullary hypertonicity, medullary Na+,K+-ATPase al, Na+,K+,2CI~ cotransporter and kidney specific chloride channel also increased during the weaning period. Those parameters were inducible by GC in infant but not in adult rats. The induction was observed after 20 min of GC exposure for Na+,K+-ATPase and after 24 h for the other transporters. Nuclear run-on studies indicated that GC directly stimulated transcription and Na+,K+-ATPase in the infant kidney. The antidiuretic hormone arginine vasopressin (AVP) regulates the urinary concentrating capacity. It is shown here that AVP regulates AQP2 on both a short- and long-term basis. AVP caused a time- and dose-dependent phosphorylation of AQP2 in rat renal papillae via the same pathway. AVP also activated the AQP2 promoter via the V2 receptor-cAMP-PKA pathway. PKA phosphorylated the cAMP-response element-binding protein (CREB) and phosphorylated CREB bound to and activated the AQP2 promoter in LLC-PK1 cells.

In conclusion, pronounced changes in the expression of water channels have been observed in the lung around birth and in the kidney during the weaning period, which suggests that they play a role for the transition from a life in fluid to a life in air. Water channels are regulated by hormones and a system by which AVP can regulate AQP2 both on a short- and long-term basis is described. These findings may be of importance for a better understanding of the pathophysiology in infants with lung edema, dehydration and volume expansion and may ultimately lead to improvement of therapy in those conditions.