No abstract

summary Sodium is absorbed in considerable amounts across the ruminal epithelium, whilst its transport is strongly interrelated with the permeation of chloride and short-chain fatty acids (SCFAs). However, regulation of ruminal Na¤, Cl¦, and SCFA absorption is hardly understood. The present study was therefore performed to characterize the influence of cAMP on sodium and sodiumcoupled transport mechanisms in short-circuited, stripped ruminal epithelia of sheep. Elevation of intracellular cAMP concentrations by theophylline (10 mÒ) or theophylline in combination with forskolin (0·1 mÒ) significantly reduced mucosal-to-serosal sodium transport, leading to a reduction of net transport. The theophylline-or theophylline-forskolin-induced reduction of sodium transport was accompanied by a decrease in chloride net transport but revealed no effect on propionate flux. Short-chain fatty acids stimulated Na¤ transport but their stimulatory effect was almost completely blocked by theophylline-forskolin. In solutions with and without SCFAs, the inhibitory effect of 1 mÒ amiloride on sodium transport was strongly reduced after theophylline-forskolin pretreatment of the tissues. Blocking the production of endogenous prostaglandins by addition of indomethacin (10 ìÒ) led to a theophylline-sensitive stimulation of unidirectional and net fluxes of sodium. The findings indicate that apical, amiloride-sensitive Na¤-H¤ exchange andÏor basolateral Na¤-K¤-ATPase can effectively be blocked by cAMP, leading to a decrease in sodium and chloride transport. In the ruminal epithelium, cAMP is a second messenger of prostaglandins, which are released spontaneously under in vitro conditions. introduction Due to the high secretion rate of the salivary glands, the amount of sodium entering the rumen of sheep and cows is 20-40 times greater than the salivary sodium load in monogastric species (Kay, 1960; Bailey, 1961a,b). The ruminal epithelium, in turn, plays an important role in maintaining the sodium balance of the animal since it is able to absorb up to 50% of the sodium secreted by the salivary glands (Dobson, 1959). However, the amount of sodium transported across the ruminal epithelium varies remarkably. An energy rich diet leads to an increase in sodium absorption, whereas food deprivation is accompanied by a sharp decrease (Gaebel et al. 1987;G abel et al. 1993). This adaptive nature of sodium transport suggests the existence of precisely controlled ion transport processes both at the systemic, hormonal level and at the cellular level.As regards control at the cellular level, studies by Wolffram et al. (1989) indicated an influence of cAMP on the transfer of sodium across the ruminal epithelium without, however, elucidating the underlying transport mechanisms. Since apical Na¤-H¤ exchange has been demonstrated to mediate a great part of ruminal sodium transport

The pre-gastric rumen of sheep plays a crucial role in the fermentation of nutrients and in the absorption of nutrients and minerals. Adrenaline has been shown previously to increase ruminal absorption of glucose and water. The present study was intended to elucidate whether ruminal ion transport is also altered by adrenaline. In Ussing chambers, changes of I(sc) were recorded in isolated ovine ruminal epithelia after the serosal additions of adrenoceptor agonists or antagonists. I(sc) increased after the addition of adrenaline (10(-4) M) or clonidine (alpha2-agonist, 10(-4) M), but decreased after the addition of isoproterenol (beta-agonist, 10(-4) M) or terbutaline (beta2-agonist, 10(-5) M). The effect of adrenaline on I(sc) was augmented by the adrenoceptor antagonists prazosin (alpha(1), 10(-4) M) and bupranolol (beta, 10(-6) M), but inversed by yohimbine (alpha(2), 10(-5) M). Adrenaline induced an increase in Na+ net flux across the epithelium that was larger than the increase in equivalent current flow. It is concluded that adrenaline differentially regulates ion transport across the ruminal epithelium via alpha1-, alpha2-, and beta2-receptors. The main effect is a stimulation of electroneutral and electrogenic Na+ absorption. This stimulated Na+ absorption might be causative of increased water absorption from the rumen as described previously.