Sighvatur S. Árnason

Steady-state sodium absorption and chloride secretion of colon and coprodeum, and plasma levels of osmoregulatory hormones in hens in relation to sodium intake

SummaryThe plasma levels of four osmoregulatory hormones and their target ion-transport systems in the lower intestines of the domestic fowl were determined in order to elucidate their interrelationship and their setpoints in relation to NaCl intake. White Plymouth Rock hens were adapted to six intake levels of NaCl (0.20±0.02–24.7±1.9 mmoles Na+·kg bw−1·day−1) for 6 weeks. The Na+ absorption and the Cl− secretion of colon and coprodeum were characterized in vitro by the effects of hexoses, amino acids, amiloride, and theophylline on the short-circuit current (SCC) and electrical potential difference (PD). The NaCl-conserving system of the adult chicken is set at low intake levels of NaCl as the 80% range (quantitized by non-linear, logistic regression analyses) of the change in the plasma [ALDO], the amiloride-inhibitable Na+ absorption of coprodeum and colon (Δ SCC), occurred from 0.18 to 2.3, from 0.9 to 4.3, and from 1.2 to 7.3 mmoles Na+·kg bw−1·day−1, respectively. These results demonstrate that the amiloride-inhibitable Na+ absorption of coprodcum is more closely linked to plasma [ALDO] than that of colon. The aminoacid-Na+ coabsorption of colon increased over exactly the same range of Na+ intake as the colonic amiloride-inhibitable Na+ absorption decreased, whereas the hexose-Na+ coabsorption increased at higher levels of Na+ intake, from 2 to 11 mmoles Na+·kg bw−1·day−1. Both these Na+ absorption types had reached their maximums at 24.7 mmoles Na+·kg bw−1·day−1, whereas the plasma [AVT] and plasma [PRL], although significantly increased, apparently had not; their 80% range of change occurred from 9.9 to 99 mmoles Na+·kg bw−1·day−1, and the main changes in plasma osmolity were predicted to occur from 5.4 to 107 mmoles Na+·kg bw−1·day−1. These results suggest that these colonic and hormonal variables conserve osmotically-free water and operate at high NaCl intake. The theophylline-induced colonic Cl− secretion did not change with NaCl intake, whereas the stimulation of SCC in coprodeum decreased with increasing NaCl intake: The main change occurred between 0 and 3.2 mmoles Na+·kg bw−1·day−1. Thus, all ion-transport capacity disappears in coprodeum with increased dietary NaCl intake, whereas colon maintains its ion-transport capacity (although the nature of the Na+ transport changes). It is suggested that hormones defending the extracellular volume and composition are regulated close to zero input and output of both NaCl and water, regardless of whether they are NaCl conserving or free-water conserving. Therefore, changes in their stable plasma concentrations occur at the extremes of tolerable range of NaCl intake.

CFTR mediated chloride secretion in the avian renal proximal tubule.

In primary cell cultures of the avian (Gallus gallus) renal proximal tubule parathyroid hormone and cAMP activation generate a Cl(-)-dependent short circuit current (I(SC)) response, consistent with net transepithelial Cl(-) secretion. In this study we investigated the expression and physiological function of the Na-K-2Cl (NKCC) transporter and CFTR chloride channel, both associated with Cl(-) secretion in a variety of tissues, in these proximal tubule cells. Using both RT-PCR and immunoblotting approaches, we showed that NKCC and CFTR are expressed, both in proximal tubule primary cultures and in a proximal tubule fraction of non-cultured (native tissue) fragments. We also used electrophysiological methods to assess the functional contribution of NKCC and CFTR to forskolin-activated I(SC) responses in filter grown cultured monolayers. Bumetanide (10 μM), a specific blocker of NKCC, inhibited forskolin activated I(SC) by about 40%, suggesting that basolateral uptake of Cl(-) is partially mediated by NKCC transport. In monolayers permeabilized on the basolateral side with nystatin, forskolin activated an apical Cl(-) conductance, manifested as bidirectional diffusion currents in the presence of oppositely directed Cl(-) gradients. Under these conditions the apical conductance appeared to show some bias towards apical-to-basolateral Cl(-) current. Two selective CFTR blockers, CFTR Inhibitor 172 and GlyH-101 (both at 20 μM) inhibited the forskolin activated diffusion currents by 38-68%, with GlyH-101 having a greater effect. These data support the conclusion that avian renal proximal tubules utilize an apical CFTR Cl(-) channel to mediate cAMP-activated Cl(-) secretion.

ALDOSTERONE AND THE CONTROL OF LOWER INTESTINAL NA+ ABSORPTION AND CL- SECRETION IN CHICKENS

There are several ion-transport systems expressed in the lower intestinal segments of chickens, depending on the level of the salt intake. The aim of this study was to test the hypothesis that aldosterone is the sole regulator of all these ion-transport systems, as had been indicated by our previous results. Chickens were long-term adapted to low salt intake, and then switched to a high-salt diet. During the first 5 days of resalination, the birds were injected with aldosterone every 8 hr and then the magnitude and characteristics of the epithelial ion-transport systems in colon and coprodeum were investigated. The results support strongly the hypothesis that aldosterone exerts major control of the amiloride-inhibitable Na(+)-transport system in both colon and coprodeum, as its magnitude was maintained high in spite of the resalination process. Spironolactone counteracted the actions of aldosterone, although not totally. On the other hand, aldosterone is not the sole regulator of the hexose/aminoacid-Na+ cotransport systems in colon, although it can act as their modulator, as the injections did delay the normal increase always seen in these transport systems during resalination. Aldosterone can also modulate the Cl(-)-secretory capacity of colon and coprodeum, but only temporarily.

Transport characteristics and morphology of the colon and coprodeum in two wild birds of different habitats, the rock ptarmigan (Lagopus mutus) and the common murre (Uria aalge)

Dietary salt intake in domestic fowl affects epithelial transport and morphology of the lower intestine (colon and coprodeum). This study investigated lower intestinal morphology and transport activity in two wild bird species with natural diets containing either low or high salt. Tissues from rock ptarmigan (Lagopus mutus) and common murres (Uria aalge) were sampled for histology and electrophysiological analyses. The ptarmigan exists on a low salt diet, while the murre lives on a high protein and high salt diet. The ptarmigan colon and coprodeum had villi/folds and crypts and the epithelium contained absorptive epithelial cells, mitochondria-rich cells and goblet cells. The colon had significant amiloride-inhibitable Isc, 5-15 μA/cm(2), with no glucose-stimulated Isc, and no significant phloridzin inhibition. The coprodeum also had high amiloride-inhibitable Isc. This transport pattern corresponded to that of chickens on low-salt diets. However, the ptarmigan colon also had a significant lysine/leucine-stimulated Isc of 3±1.0 μA/cm(2). The short U. aalge colon was similar to that of ptarmigans, but with no villi. It demonstrated a significant lysine/leucine-stimulated Isc (11±3.5 μA/cm(2)) with no amiloride-inhibitable Isc, similar to the high-salt chicken colon, but with no Na(+)-glucose cotransport. The murre coprodeum was inert to all substances and showed high resistance (1000 Ω·cm(2)), with a multilayered squamous epithelium. Despite some variations possibly associated with dietary protein intake, we conclude that natural high and low salt diets in different avian species are associated with different lower intestinal transport patterns, providing for post-renal adjustments in ion and water excretion.