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Biomedical subjects

T C Chu

Publications and source records attributed to T C Chu.

At least 73 records · Page 4Linked to original sources
PubMed1987

Electrical parameters of the isolated monkey ciliary epithelium and effects of pharmacological agents.

The electrical properties of the isolated monkey ciliary epithelium (CE) were determined in an Ussing-type chamber. In a Hepes, HCO3- buffered solution, transepithelial potential difference (PD), short-circuit current (SCC) and electrical resistance (Rt) were -2.5 mV (aqueous-side negative), 8.5 microA and 246 omega, respectively. Epinephrine (0.01 mM) increased the SCC and PD across the isolated monkey CE when added to the aqueous-side bathing solution but was without effect when added to the blood-side bathing solution. Forskolin (0.01 mM) increased the SCC and PD when added to the bathing solution on either side. A disulfonic stilbene, DIDS (0.1 mM), reduced the SCC and PD when added to the aqueous-side bathing solution but was without effect when added to the blood-side bathing solution. Ouabain (0.1 mM) added to the aqueous-side produced a transient stimulation followed by a gradual inhibition of the SCC and PD. On the blood-side, ouabain directly inhibited the SCC and PD towards zero. Although the general electrical properties were similar to those of the isolated rabbit iris-ciliary epithelium, there were differences in the effects of these pharmacological agents on the electrical properties.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗
PubMed1986

Forskolin effects on frog and rabbit corneal epithelium ion transport.

The effects of forskolin on the electrophysiological parameters of the isolated corneal epithelium from bullfrog (Rana catesbeiana) were investigated. Forskolin stimulated the short-circuit current (SCC) and transepithelial potential difference (PDt), while reducing the transepithelial resistance. These effects were absent in Cl- -free bathing solutions. Furosemide, added either before or after forskolin, completely blocked the effects. Epinephrine and A23187, added after forskolin, produced only a small additional stimulation of the SCC. Propranolol neither blocked nor reduced the effect of forskolin. Forskolin increased the stroma to tear 36Cl flux by 61% and the tear to stroma 36Cl flux by 64%. Intracellular recordings showed that forskolin depolarized the potential difference across the apical membrane and reduced the apical/basolateral resistance ratio. Intracellular recordings in the isolated rabbit epithelium showed the same effects by forskolin except that there was only a brief stimulation of PDt, after which it stabilized slightly below the control level. These results are consistent with an increase in apical membrane permeability similar to that produced by adenosine 3',5'-cyclic monophosphate, epinephrine, and the Ca2+ ionophore A23187.

Animals↗
PubMed1986

Effects of forskolin, prostaglandin F2 alpha, and Ba2+ on the short-circuit current of the isolated rabbit iris-ciliary body.

To gain information on the role of cyclic AMP (cAMP), ion transport and cell membrane permeability on aqueous humor formation, agents with well-known effects on transport properties in other epithelia were tested on the isolated rabbit iris-ciliary body. Forskolin stimulated the short-circuit current (SCC) by 37.5% when added to the aqueous-side solution. Forskolin was ineffective when added to the blood-side solution or when HCO3- was absent from the bathing solutions. The effect of forskolin confirms the presence of adenylate cyclase in the ciliary epithelium and the involvement of cAMP in ion transport. In HCO3- -rich media, 5 X 10(-5) M prostaglandin F2 alpha (PGF2 alpha), produced a prompt 25% increase in the SCC when added to the aqueous-side, and a small stimulatory SCC response when added to the blood-side. No change in SCC occurred when PGF2 alpha was added to either side of a HCO3- -free bathing solution. It is implied that cAMP acts on a HCO3- dependent transport system. These results are consistent with the previously observed stimulation of the SCC by 8Br-cAMP. BaCl2, 2.5 mM, on the aqueous-side increased the SCC by 240.5%, but reduced the SCC by 26% when added to the blood-side solution. The Ba2+ effects indicate the presence of high conductance K+ channels in the basolateral membranes of both the pigmented and non-pigmented cell layers.

Animals↗
PubMed1985

Effects of adrenergic agonists and cyclic AMP on the short-circuit current across the isolated rabbit iris-ciliary body.

Epinephrine, isoproterenol and norepinephrine (10(-6) M - 10(-4) M) reduced the short-circuit current (SCC) across the isolated rabbit iris-ciliary body when added to the blood side (pigmented layer) bathing solution but were without effect when added to the aqueous side (non-pigmented layer) bathing solution. The effect was observed in both HCO-3-rich and HCO-3-free solutions, although it was smaller in the latter. Propranolol and phentolamine (10(-5) M) partially blocked the action of the adrenergic agonists, 8Br-cAMP (1 mM), on the other hand, stimulated the SCC when added to either bathing solution. The apparent contradiction between these results is explained on the basis of separate cyclic AMP compartmentalization in the two epithelial cell layers and the lack of adrenoceptors in the non-pigmented layer.

8-Bromo Cyclic Adenosine Monophosphate↗
PubMed1983

Mechanisms responsible for SCN increase in resistance of in vitro frog gastric mucosa.

Thiocyanate (SCN) inhibits H+ secretion and increases the resistance and potential difference (PD) of the gastric mucosa. These results support our separate-site electrogenic theory of HCl secretion. Recent work shows that an ATP-driven mechanism in the gastric mucosa can produce H+ by a neutral exchange of K+ for H+. The SCN increase in resistance and PD, if due to an inhibition of a high-conductance mechanism(s) in the secretory plasma membrane, is not easily compatible with the neutral mechanism. Therefore, the possibility was examined that SCN induces the increase in resistance by other mechanisms. The HCl and NaCl concentration profiles in the pit and tubular lumina were calculated. The effects of SCN were determined with isotonic, hypotonic, hypertonic, buffered, and high [H+] secretory solutions. The results indicate that SCN produces an increase in resistance of about 130 omega. cm2 of the plasma membranes of the tubular cells. A scheme is proposed that incorporates the neutral K+-H+ mechanism into an electrogenic system.

Animals↗
PubMed1982

Evidence for electrogenic Na-Cl symport in the in vitro frog stomach.

The transmucosal potential difference (PD) response to step changes in Na+ (choline or Mg2+ for Na+) in the nutrient fluid was studied. Decreasing the Na+ concentration in the nutrient fluid decreased the positivity of the nutrient side, and increasing the Na+ concentration increased the positivity. This effect is anomalous because it is opposite to that of a simple Na+ conductive pathway but right for a choline or Mg2+ conductive pathway. The lack of linearity of delta PD versus the log of the choline concentration ruled out a choline conductive pathway. An approximate linear relation was obtained for Na+. The anomalous PD response for Na+ was reduced but not abolished in 1) K+-free bathing fluids and 2) 10(-3) M ouabain in nutrient fluid. Changing the Cl- concentration in nutrient fluid gave normal PD responses, which were reduced in the absence of Na+. These results are attributed to a passive electrogenic Na-Cl symport. Theoretical considerations, for p Na+ and q Cl- transported per cycle (with q greater than p), predict an anomalous delta PD for Na+ concentration changes and a normal delta PD for Cl- concentration changes.

Animals↗
PubMed1981

Anomalous potential response and (Na+ + K+)-ATPase in in vitro frog gastric mucosa.

In general, increasing K+ on the nutrient side decreases the transmucosal PD (nutrient becomes more negative) but after bathing the mucosa in zero K+ media for about 30 min, or longer, elevation of K+ on the nutrient side increases the PD, an anomalous effect. In Cl- media, increasing nutrient K+ from zero to 4 mM produces an increase in PD (an anomalous response) of 3.1 and 5.3 mV in 2 and 5 min, respectively. Ouabain (10(-3) M) to the nutrient side abolished the anomalous response as did removal of Na+ (choline for Na+) from bathing media. In SO4(2-) media (SO4(2-) for Cl-), a significant anomalous PD response was observed when K+ on the nutrient side was increased from zero to 1, 2 or 3 mM but not to higher K+ concentrations. In this case, ouabain also abolished the anomalous response. It is postulated, on the basis of the effects of ouabain and the use of choline media, that an electrogenic (Na+ + K+)-ATPase pump is present on the nutrient-facing membrane in which more Na+ than K+ are transported per cycle.

Animals↗
PubMed1981

Effects of ouabain on frog gastric mucosa in vitro.

The effect of the addition of ouabain to the nutrient solution was determined on resistance, potential difference (p.d.) and H+ secretion rate. In NaCl media, 10(-3) M ouabain decreased significantly the p.d. from 25.6 mV to 16.1 mV in 30 min and to 11.0 mV in 60 min. No significant changes occurred in resistance and H+ secretion rate. In Na2SO4 (Cl(-)-free) media, ouabain produced a biphasic effect on p.d. The p.d. changed from -28.0 mV (nutrient-negative) to a nadir of -37.4 mV in 7 min and then increased to -16.4 mV in 60 min. At the nadir there was no significant change in resistance or H+ secretion rate but at 60 min, unlike Cl- media, resistance increased by 36% and H+ secretion rate decreased by 43%. To decide whether the ouabain-caused decrease in H+ rate in Na2SO4 media was due to an effect on the H+ pump or on resistance of the return pathways, the voltage was clamped at 0 and 40 mV. Clamping the voltage showed that in the case of a marked decrease in the H+ secretion rate, the H+ transport mechanism itself was inhibited (and not the parallel pathway). The decrease in p.d. due to ouabain in Cl- and SO42- media indicates that the (Na+ + K+)-ATPase mechanism may be electrogenic.

Animals↗
PubMed1981

Origins of positive potential difference of frog gastric mucosa in Cl--free solutions.

Experiments were performed on gastric mucosae of Rana pipiens with an in vitro method. Bathing both sides of the mucosae in Cl--free solutions resulted in an inverted potential difference (PD) (i.e., nutrient negative). Cimetidine in a concentration of 10(-4) or 10(-3) M in the nutrient solution gave a positive PD when the H+ secretory rate was zero or near zero and confirmed that the PD versus H+ rate was linear. the origins of the positive PD were examined by ion replacement, e.g., choline for Na+ in both solutions. When both solutions had 25 mM HCO(-)3 (i.e., no HCO(-)3 gradient), the difference in PD between choline sulfate and Na2SO4 solutions indicated a possible small active transport of Na+ from the secretory to nutrient side. Similar experiments indicated no active K+ transport. A residual PD occurred without Na+. With H+ rate zero, 100% O2, and no HCO(-)3, a base secretion of about 0.4 mueq.h-1.cm-2 was obtained in a direction to contribute to the positive PD. Active SO2(-)4 transport may occur.

Animals↗