Net Calcium Secretion Research Articles

Study of acute pharmacologic effects of prolactin on calcium and water transport in the rat colon by an in vivo perfusion technique

We investigated the acute effect of intraperitoneally administered prolactin on calcium and water transport in colon of sexually mature female Wistar rats using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 (microCi 45CaCl2 (1 Ci = 37 GBq) was perfused througth the 8-cm colonic loop for 60 min at perfusion rates of 0.5 or 1.0 mL x min(-1). Calcium and water transport was also studied under a no flow condition to stimulate the condition often found in the colon by in vivo ligated colonic loop for 30 min. Control results showed no correlation between calcium transport and water flux. Flow of luminal solution at 0.5 and 1.0 mL x min(-1) was found to reverse net calcium absorption from 0.04+/-0.01 nmol x g(-1) dry weight x h(-1) to net calcium secretion of 0.04+/-0.04 and 0.9+/-0.02 nmol x g(-1) dry weight x h(-1), respectively. Neither 0.4, 0.6, nor 1.0 mg x kg(-1) prolactin had any effect on calcium fluxes in the colon. On the other hand, at a perfusion rate of 1 mL x min(-1), 0.4 mg x kg(-1) prolactin significantly decreased net water absorption from 3.86+/-0.90 to 0.88+/-0.64 mL x g(-1) dry weight x h(-1) (P < 0.001), and the higher doses of 0.6 and 1.0 mg x kg(-1) prolactin reversed net water absorption to net water secretion of 2.20+/-0.63 and 2.33+/-0.89 mL x g(-1) dry weight x h(-1), respectively (P < 0.001). The stimulatory effect of prolactin on water transport was completely abolished by reducing the perfusion rate from 1.0 mL x min(-1) to zero. The stimulatory effect of prolactin on water secretion at perfusion rate of 1.0 mL x min(-1) was also abolished when luminal [Na+] was reduced from 180 to 80 mM. We concluded that, unlike in the small intestine, calcium fluxes in the colon are not related to water transport and did not respond at all to prolactin. Water transport, on the other hand, was reversed from net absorption to secretion by prolactin. We propose that this prolactin-induced water secretion is probably mediated by recycling of luminal sodium in the vicinity of tight junctions.

Study of acute pharmacologic effects of prolactin on calcium and water transport in the rat colon by an in vivo perfusion technique

We investigated the acute effect of intraperitoneally administered prolactin on calcium and water transport in colon of sexually mature female Wistar rats using an in vivo perfusion technique. Test solution containing (in mM) NaCl, 100; KCl, 4.7; MgSO4, 1.2; CaCl2, 20; D-glucose, 11; sodium ferrocyanide (Na4Fe(CN)6), an index of net water transport, 20; and 0.7 (microCi 45CaCl2 (1 Ci = 37 GBq) was perfused througth the 8-cm colonic loop for 60 min at perfusion rates of 0.5 or 1.0 mL x min(-1). Calcium and water transport was also studied under a no flow condition to stimulate the condition often found in the colon by in vivo ligated colonic loop for 30 min. Control results showed no correlation between calcium transport and water flux. Flow of luminal solution at 0.5 and 1.0 mL x min(-1) was found to reverse net calcium absorption from 0.04+/-0.01 nmol x g(-1) dry weight x h(-1) to net calcium secretion of 0.04+/-0.04 and 0.9+/-0.02 nmol x g(-1) dry weight x h(-1), respectively. Neither 0.4, 0.6, nor 1.0 mg x kg(-1) prolactin had any effect on calcium fluxes in the colon. On the other hand, at a perfusion rate of 1 mL x min(-1), 0.4 mg x kg(-1) prolactin significantly decreased net water absorption from 3.86+/-0.90 to 0.88+/-0.64 mL x g(-1) dry weight x h(-1) (P < 0.001), and the higher doses of 0.6 and 1.0 mg x kg(-1) prolactin reversed net water absorption to net water secretion of 2.20+/-0.63 and 2.33+/-0.89 mL x g(-1) dry weight x h(-1), respectively (P < 0.001). The stimulatory effect of prolactin on water transport was completely abolished by reducing the perfusion rate from 1.0 mL x min(-1) to zero. The stimulatory effect of prolactin on water secretion at perfusion rate of 1.0 mL x min(-1) was also abolished when luminal [Na+] was reduced from 180 to 80 mM. We concluded that, unlike in the small intestine, calcium fluxes in the colon are not related to water transport and did not respond at all to prolactin. Water transport, on the other hand, was reversed from net absorption to secretion by prolactin. We propose that this prolactin-induced water secretion is probably mediated by recycling of luminal sodium in the vicinity of tight junctions.

Micropuncture study of the avian kidney: infusion of mannitol or sodium chloride

Renal function was studied in anesthetized starlings by micropuncture and clearance techniques. The effects of intravenous infusions of isosmotic sodium chloride and hyposmotic mannitol solutions were compared. In the proximal tubules of the superficial reptilian-type nephrons, there is net reabsorption of sodium, chloride, and potassium; net secretion of calcium and urate; and either net secretion or no net secretion or reabsorption of magnesium and phosphate. The fractional delivery of ions and urate to the site of micropuncture, the percent of fluid reabsorbed, and the single nephron glomerular filtration rate (SNGFR) were not significantly different between the two infusion treatments. However, in the sodium chloride-infused birds, whole kidney GFR and the fractional excretion of sodium and chloride by the whole animal were higher, and fractional excretion of magnesium and potassium were lower. The data for each infusion group were further subdivided on the basis of whether the birds were in molt. In starlings, molt is associated with elevated levels of plasma prolactin. For the sodium chloride infusion, sodium and water reabsorption to the site of micropuncture was greater in the birds in molt than in those not in molt.

Cellular and paracellular calcium transport in the rat ileum and the influence of 1 alpha, 25-dihydroxyvitamin D3 and dexamethasone.

Concentration dependence of unidirectional calcium fluxes across the rat ileum freed from the serosa and the muscularis externa were measured in a modified Ussing-chamber. Mucosa (m) to serosa (s) calcium flux showed a saturable component, whereas s to m calcium flux was linearly related to the calcium concentration between 0.125 mmol/l and 5 mmol/l. At all calcium concentrations used net secretion of calcium was observed. The s to m flux of the simultaneously measured paracellular marker mannitol at all calcium concentrations was remarkably higher than the m to s flux, resulting in net mannitol secretion. The results obtained from the calcium fluxes when clamping the transepithelial electrical potential agree well with those of the concentration dependence of the calcium fluxes: 1. Only m to s flux has a voltage independent, transcellular component. 2. Calcium s to m flux is totally voltage dependent, i.e. diffusive. 3. Diffusional s to m calcium flux is about 80% greater than the diffusional fraction of the m to s flux. Omitting glucose from the bathing solution effected a decrease of the transepithelial electrical potential and of the short circuit by 91% and 85% respectively; net calcium secretion was almost abolished and net mannitol secretion remarkably reduced. Addition of glucose, which stimulates water absorption in the ileum as a metabolic substrate, activated m to s but significantly more pronounced s to m calcium flux parallel to that of mannitol.(ABSTRACT TRUNCATED AT 250 WORDS)

Thiazides stimulate calcium absorption in urinary bladder of winter flounder

Thiazides inhibit voltage-independent NaCl absorption in the urinary bladder of the winter flounder presumably by blocking an electroneutral mucosal Na/Cl co-transporter. As thiazides stimulate calcium absorption in mammalian distal convoluted tubule while inhibiting NaCl absorption, we studied the effects of hydrochlorothiazide (HCTZ) on unidirectional 45Ca fluxes and intracellular electrical potential in short-circuited bladders to examine possible mechanisms of HCTZ effects on calcium transport. Basal secretory calcium flux was, on average, slightly larger than absorptive flux, reflecting small net calcium secretion. Mucosal addition of HCTZ (10 −4 M) stimulated absorptive calcium flux by 46% while the secretory flux was unaltered. Thus, HCTZ tended to induce net calcium absorption. Pre-treatment with serosal ouabain (10 −4 M) attenuated the HCTZ-induced increase in absorptive calcium flux. Moreover, HCTZ hyperpolarized the mucosal membrane potential by 18% as measured by conventional open-tip microelectrodes. These effects of HCTZ are consistent with the hypothesis that HCTZ indirectly stimulates Na/Ca exchange located at the serosal membrane. In conclusion, HCTZ in flounder urinary bladder, as in mammalian distal convoluted tubule, simultaneously inhibits NaCl absorption and stimulates calcium absorption. This study expands on the functional similarities between the flounder urinary bladder and the mammalian distal convoluted tubule.

The role of the paracellular pathway in the net transport of calcium across the colonic mucosa.

Concentration dependent calcium fluxes across the colon descendens of the rat were measured in a modified Ussing chamber. Mucosa (m) to serosa (s) calcium flux showed a saturable component, whereas s to m calcium flux was linearly related to the calcium concentration. At low calcium concentrations net absorption and at concentration above 2.5 mmol/l net secretion of calcium was observed. The results obtained from the unidirectional calcium fluxes when clamping the transepithelial electrical potential agree well with those of the concentration dependence of the calcium fluxes: Only m to s flux has a voltage independent component. Calcium s to m movement is totally voltage dependent. Diffusional s to m calcium flux is greater than the diffusional fraction of the m to s calcium flow. Dexamethasone, known to stimulate water absorption in the colon descendens by an activation of sodium transport, had no effect on the cellular mediated m to s calcium transport but significantly increased paracellular s to m flux parallel to that of the extracellular marker mannitol. This increase in paracellular s to m calcium and mannitol flux was completely abolished by amiloride, which is known to suppress the dexamethasone-induced stimulation in sodium and water absorption. The results demonstrate that the increased paracellular s to m calcium and mannitol flow is oppositely directed to the dexamethasone-induced net fluid movement as it could be expected on the basis of Ussing's "anomalous solvent drag" effect.

Direct effects of calcium channel blockers on duodenal calcium transport in vivo

The direct effects of verapamil, diltiazem and nifedipine on duodenal calcium transport were assessed in rats by the in vivo ligated loop technique, using luminal calcium concentrations at which active and passive transport mechanisms predominate (2 and 50 mM Ca, respectively). At 2 mM Ca, addition of verapamil (0.3–10 mM) to the luminal solution caused a concentration-dependent decrease in calcium lumen-to-plasma uptake and an increase in calcium plasma-to-lumen translocation, such that at 10 mM verapamil there was a net secretion of calcium into the duodenal lumen. In contrast, nifedipine (0.3–3 mM) was without effect on calcium transport, and diltiazem reduced calcium lumen-to-plasma uptake and net calcium absorption only at 10 mM, without influencing plasma-to-lumen translocation. The verapamil-induced increase in calcium plasma-to-lumen translocation was abolished by bile duct ligation. Calcium transport was unaffected by any calcium channel blocker at 50 mM luminal calcium. Thus, verapamil can directly influence active calcium translocation in the intestine, in vivo, and may affect calcium homeostasis during chronic oral treatment with this drug.

Acute effect of ethanol on intestinal calcium transport.

In vivo studies in rats demonstrated that acute intragastric administration of ethanol (3 g/kg BW) resulted in a reduced net absorption of fluid and calcium. Thirty minutes after an i.v. injection of 2 microCi 45Ca, the gastrointestinal 45Ca content in ethanol-treated rats was significantly higher than in controls indicating an increase in calcium secretion. The effect of ethanol on calcium movement was quantitated by measuring calcium fluxes across the in situ duodenal and ileal loops. Ethanol administration suppressed net duodenal calcium absorption by inhibiting the lumen to plasma flux of calcium. In contrast, ethanol enhanced plasma to lumen calcium flux in the ileum, resulting in net calcium secretion.

Voltage-dependent calcium movement across the cortical collecting duct.

Cortical collecting ducts were dissected from rabbit kidneys and perfused in vitro. Unidirectional transepithelial calcium fluxes from bath-to-lumen and lumen-to-bath were measured with 45Ca. Transepithelial voltage was varied over a wide range by pharmacologic manipulations. With lumen-negative voltages net calcium secretion from bath to lumen, which varied directly with the voltage, was observed. At voltages near 0 there was no measurable net flux. When the voltage was made positive, the direction of net calcium transport reversed (i.e., absorption from lumen to bath). Calcium permeability, calculated from the dependence of net flux on voltage, was 1.4 X 10(-7) cm/s, which is less than 2% of the calcium permeability previously determined in the cortical thick ascending limb of Henle's loop. Analysis of the calcium flux ratios revealed apparent interdependence of the bidirectional fluxes consistent with exchange diffusion but no evidence for active calcium transport. We conclude that there is a small, but measurable, component of passive net calcium flux driven by the transepithelial voltage across the cortical collecting duct.

Intestinal maturation: effect of luminal osmolality on net mineral secretion.

We investigated with an in vivo single-pass perfusion technique net secretion of calcium, magnesium, and zinc from proximal and distal small intestinal segments and from the cecum plus colon segments of suckling, weanling, and adolescent rats during perfusion with either isotonic (300 mOsm/kg) or hypertonic (500 mOsm/kg) mineral-free solutions. There was not secretion of calcium, magnesium, and zinc in all segments perfused at all age periods. During isotonic perfusion, rates of net secretion of calcium and zinc were severalfold greater in the proximal and distal segments of the suckling rats compared with the corresponding segments of the adolescent rats. Net magnesium secretion was similar in the sucking and adolescent rats. Net secretion rates for the weanling rats tended to be intermediate. During hypertonic perfusion, net secretion rates for calcium, magnesium, and zinc were severalfold greater from all segments of the suckling rats compared with the corresponding segments of the adolescent rats. During either isotonic or hypertonic perfusions, rates of net secretion of calcium and magnesium in general were greater from the proximal and distal segments than from the cecum plus colon in the suckling and weanling rats. In contrast, for the adolescent rats, net secretions from all segments in general were similar. For zinc, rates of net secretion were somewhat greater in the cecum plus colon of rats in all age groups. Our findings suggest a greater permeability of the intestinal epithelium, not only to water and electrolytes but also to certain minerals (calcium and magnesium) and trace elements (Zn) in suckling rats compared with adolescent rats. The implication is that during periods of osmotic diarrhea, infant animals appear to be at risk for losing excessive amounts of minerals and trace elements. These findings may have clinical relevance to human infants suffering from recurrent diarrheal diseases.

Intestinal Resection and Calcium Absorption in the Rat

SummaryCalcium transport was measured in duodenum and ileum of young rats 7-9 days post 70% resection of mid-small intestine. Duodenal calcium absorption per unit weight of mucosa was depressed to half that of transected controls because of decreased flux out of the lumen. Since absorption per cm segment length of duodenum was also depressed in resected as compared with transected animals, the mucosal hyperplasia secondary to resection did not compensate for decreased absorption per unit weight of mucosa. Ileum of resected animals showed net calcium secretion in comparison with net absorption in transected controls. Possible causes for depression of calcium absorption include abnormal vitamin D metabolism with decreased enterohepatic cycling of polar vitamin D metabolites or immaturity of rapidly proliferating mucosal cells.We thank Fred Kirchner, Radiation Research Laboratory, University of Iowa, for advice regarding the intestinal resections.

Intestinal Calcium Loss in Infant Rats

SummarySegments of the jejunum and ileum of 14-15-day-old and 42-day-old rats were perfused in situ with isotonic or hypertonic calcium free solutions. There was net secretion of calcium in all segments perfused. Rates of net secretion of calcium were several fold and significantly greater in segments of the 14- to 15-day-old than in corresponding segments of the 42-day-old rats. In the 14- to 15-day-old rats net secretion of calcium was similar in jejunal and ileal segments, while in the 42-day-old rats net secretion was twofold greater in the ileal than jejunal segments. Perfusion of the hypertonic solution almost doubled net secretion of calcium in the jejunal segments at both ages, but had no effect on rate secretion of calcium in the ileal segments. These findings suggest that infants are more prone to lose calcium into the lumen of the intestine than adults, especially during episodes of osmotic diarrhea when oral intake is restricted acutely.

Anomalous calcium secretion in rat ileum: role of paracellular pathway.

The mechanism of apparent calcium secretion by unstripped rat ileum in vitro has been investigated using mannitol and polyethylene glycol (mol wt 900) as markers for extracellular transepithelial flux. In the absence of electrochemical gradients between the mucosal and serosal bathing media in a modified Ussing chamber, net fluxes of both mannitol and polyethylene glycol were observed from serosa to mucosa in the presence of 11 mM D-glucose. Mucosal-to-serosal calcium flux reveals a significant cellular component, but serosal-to-mucosal calcium permeability is a linear function of mannitol permeability, suggesting an exclusively extracellular route. For the mucosal-to-serosal fluxes, inhibition of calcium flux by 1 mM N-ethylmaleimide results in a calcium-to-mannitol permeability ratio indistinguishable from that measured for serosal-to-mucosal flux. This evidence suggests that the apparent calcium secretion observed at 10 mM medium calcium is not the result of a cellular secretory mechanism. It is proposed that a hydrostatic driving force generated internal to the tissue but external to the cells results in net calcium secretion at calcium conentrations that saturate the cellular absorptive mechanism.

Duodenal and ileal adaptation to dietary calcium restriction: in vivo studies in the rat.

Intestinal adaptation by the growing rat to a low-calcium diet was studied by in situ perfusion of duodenum and ileum in vivo. Rats were fed diets containing either 1.2 or 0.02% Ca for 17-24 days. To study plasma-to-lumen flux and net calcium absorption, rats were loaded parenterally with 45Ca and perfused intraluminally with 3.4 mM calcium. Calcium restriction caused net absorption in ileum to increase fourfold, in duodenum almost twofold. With calcium restriction, plasma-to-lumen flux decreased in duodenum but not in ileum and was small relative to absorption. However, net calcium secretion, when measured in a separate set of animals by intraluminal perfusion of NaCl, decreased in ileum but not in duodenum in response to calcium restriction. The magnitude of adaptation is greater in ileum than duodenum and is largely the result of increased lumen-to-plasma flux. The distal small intestine is probably crucial for calcium homeostasis in dietary calcium deficiency.