No abstract

We have designed and synthesized benzo[c]quinolizinium derivatives and evaluated their effects on the activity of G551D cystic fibrosis transmembrane conductance regulator (CFTR) expressed in Chinese hamster ovary and Fisher rat thyroid cells. We demonstrated, using iodide efflux, whole cell patch clamp, and short-circuit recordings, that 5-butyl-6-hydroxy-10-chlorobenzo[c]quinolizinium chloride (MPB-91) restored the activity of G551D CFTR (EC(50) = 85 microM) and activated CFTR in Calu-3 cells (EC(50) = 47 microM). MPB-91 has no effect on the ATPase activity of wild-type and G551D NBD1/R/GST fusion proteins or on the ATPase, GTPase, and adenylate kinase activities of purified NBD2. The activation of CFTR by MPB-91 is independent of phosphorylation because 1) kinase inhibitors have no effect and 2) the compound still activated CFTR having 10 mutated protein kinase A sites (10SA-CFTR). The new pharmacological agent MPB-91 may be an important candidate drug to ameliorate the ion transport defect associated with CF and to point out a new pathway to modulate CFTR activity.

The pharmacological activation of the cystic fibrosis gene protein cystic fibrosis transmembrane conductance regulator (CFTR) was studied in human airway epithelial Calu-3 cells, which express a high level of CFTR protein as assessed by Western blot and in vitro phosphorylation. Immunolocalization shows that CFTR is located in the apical membrane. We performed iodide efflux, whole cell patch-clamp, and short-circuit recordings to demonstrate that the novel synthesized xanthine derivative 3, 7-dimethyl-1-isobutylxanthine (X-33) is an activator of the CFTR channel in Calu-3 cells. Whole cell current activated by X-33 or IBMX is linear, inhibited by glibenclamide and diphenylamine-2-carboxylate but not by DIDS or TS-TM calix[4]arene. Intracellular cAMP was not affected by X-33. An outwardly rectifying Cl(-) current was recorded in the absence of cAMP and X-33 stimulation, inhibited by DIDS and TS-TM calix[4]arene. With the use of short-circuit recordings, X-33 and IBMX were able to stimulate a large concentration-dependent CFTR transport that was blocked by glibenclamide but not by DIDS. Our results show that manipulating the chemical structure of xanthine derivatives offers an opportunity to identify further specific activators of CFTR in airway cells.

The ability of ATP and FSH to induce intracellular calcium [Ca(2+)](i) changes in Sertoli cells is imperfectly understood and reports are conflicting. We have applied the single-cell microfluorometry technique with the calcium probe indo-1 to investigate [Ca(2+)](i) in individual cultured Sertoli cells. When cells were exposed to ATP, cAMP, and FSH, a fast and biphasic increase in [Ca(2+)](i) was obtained in 100%, 70%, and 56% of cells, respectively. Caffeine did not activate Ca(2+) mobilization, while thapsigargin suppressed the peak response. External calcium free-EGTA buffer suppressed the plateau phase, while blockers of voltage-operated Ca(2+) channels did not abolish the response to cAMP and ATP. We conclude that the three messengers mobilized Ca(2+) from intracellular thapsigargin-sensitive stores, which induced a subsequent Ca(2+) influx from the extracellular medium by a voltage-independent Ca(2+) entry. The well-documented mechanisms by which these messengers act on cells support the idea that they release Ca(2+) from smooth endoplasmic reticulum by two different pathways, or that FSH and cAMP first release ATP, which then acts on cells. Among the cells, 77% and 80% responded, respectively, to FSH and cAMP by a delayed long-lasting decrease in [Ca(2+)](i) that was never recorded in the presence of ATP. This suggests that FSH and cAMP also promote a slow redistribution of [Ca(2+)](i) from the exchangeable pool to the bound nonexchangeable pools. Involvement of voltage-operated and voltage-independent calcium channels in the response of Sertoli cells to ATP, FSH, and cAMP is discussed.

SUMMARY1. The effect of FSH on the membrane potential of Sertoli cells has been investigated by intracellular microelectrode recording from monolayer cultures of cells, isolated from immature rat testes by enzymatic treatment.2. In standard Earle's solution, the membrane potential of unstimulated cells in a 3-day-old monolayer was -21-6 + 0-2 mV (n = 300). The recorded potentials were unchanged on varying the culture period from 3 to 7 days or by removal of chloride or calcium from the media. However, they were decreased in a low-sodium or potassium-rich medium.3. Ovine FSH caused hyperpolarization of-the cell in a dose-dependent manner. Maximal values were obtained with concentrations ranging between 2-9 and 59,ug/ml (-37'0 +02 mV; n = 310). Dibutyryl cyclic AMP (1 mM) produced a similar effect to FSH. Under similar conditions human chorionic gonadotrophin (hCG) had no effect on the membrane potential.4. The FSH-induced hyperpolarization was unchanged by chloride or bicarbonate replacement. It was decreased by low-sodium media (9 mM) (-29-6+ 0-3 mV; n = 110), by removal of calcium (-32-4+0-6 mV; n = 128) and by an increase in the potassium concentration of the bathing medium. A tenfold increase in potassium concentration depolarized the cell by 29-5 mV against 16 mV for unstimulated cells.5. FSH-induced hyperpolarization was decreased by application of ouabain (10-4 M), quinidine (1-4 x 10-4 M) and cobalt (10-3 M) (respectively: -28-7 + 0 3 mV, n = 124; -26-6+003 mV, n = 52 and -24-8+0-3 mV, n = 68) but was unchanged by amiloride (10-4M) and TTX (3 x 10-6M). None of these drugs modified the membrane potential of unstimulated cells.6. All these results suggest that FSH stimulation of Sertoli cells in monolayer culture involves the modification of their membrane permeabilities.