Evaluation of in Vitro Rat and Human Airway Epithelial Models for Acute Inhalation Toxicity Testing

In vivo models (mostly rodents) are currently accepted by regulatory authorities for assessing acute inhalation toxicity. Considerable efforts have been made in recent years to evaluate in vitro human airway epithelial models (HAEM) as replacements for in vivo testing. In the current work, an organotypic in vitro rat airway epithelial model (RAEM), rat EpiAirway™, was developed and characterized to allow a direct comparison with the available HAEM, human EpiAirway™, in order to address potential interspecies variability in responses to harmful agents. The rat and human models were evaluated in two independent laboratories with 14 reference chemicals, selected to cover a broad range of chemical structures and reactive groups, as well as known acute animal and human toxicity responses, in three replicate rounds of experiments. Toxicity endpoints included changes in tissue viability (MTT assay), epithelial barrier integrity (TEER, transepithelial electrical resistance), and tissue morphology (histopathology). The newly developed rat EpiAirway™ model produced reproducible results across all replicate experiments in both testing laboratories. Furthermore, a high level of concordance was observed between the RAEM and HAEM toxicity responses (determined by IC25) in both laboratories, with R2 = 0.78 and 0.88 when analyzed by TEER; and R2 = 0.92 for both when analyzed by MTT. These results indicate that rat and human airway epithelial tissues respond similarly to acute exposures to chemicals. The new in vitro RAEM will help extrapolate to in vivo rat toxicity responses and support screening as part of a 3Rs program.

Increasing Cellular Uptake and Permeation of Curcumin Using a Novel Polymer-Surfactant Formulation

Several therapeutically active molecules are poorly water-soluble, thereby creating a challenge for pharmaceutical scientists to develop an active solution for their oral drug delivery. This study aimed to investigate the potential for novel polymer-surfactant-based formulations (designated A and B) to improve the solubility and permeability of curcumin. A solubility study and characterization studies (FTIR, DSC and XRD) were conducted for the various formulations. The cytotoxicity of formulations and commercial comparators was tested via MTT and LDH assays, and their permeability by in vitro drug transport and cellular drug uptake was established using the Caco-2 cell model. The apparent permeability coefficients (Papp) are considered a good indicator of drug permeation. However, it can be argued that the magnitude of Papp, when used to reflect the permeability of the cells to the drug, can be influenced by the initial drug concentration (C₀) in the donor chamber. Therefore, Papp (suspension) and Papp (solution) were calculated based on the different values of C₀. It was clear that Papp (solution) can more accurately reflect drug permeation than Papp (suspension). Formulation A, containing Soluplus^® and vitamin E TPGs, significantly increased the permeation and cellular uptake of curcumin compared to other samples, which is believed to be related to the increased aqueous solubility of the drug in this formulation.

The effect of ingredients commonly used in nasal and inhaled solutions on the secretion of mucus in vitro

Hypersecretion of mucus is associated with impaired mucociliary clearance that can influence the retention of active pharmaceutical ingredients in the airway but is also linked with recurrent airway disease. Therefore, the effect on mucin secretion of a range of ingredients used in solutions delivered to the nose and lung was studied. Mucin secretion from explants of ovine epithelium was quantified using an enzyme-linked lectin assay (ELLA) or sandwich ELLA depending on the compatibility of the ingredients with the assay. Benzalkonium chloride (0.015% w/w), Methocel™ E50 premium LV (1.0% w/w), propylene glycol (1.5% w/w), potassium sorbate + propylene glycol (0.3% w/w + 1.5% w/w) and polysorbate 80 (0.025% w/w), used at common working concentrations, all increased the secretion of mucin from the explants (P < 0.05). Ethylenediamine tetraacetic acid-disodium salt (EDTA) (0.015% w/w), Avicel® RC591 (1.5% w/w), fluticasone furoate (0.0004% w/w, concentration in solution) and dimethyl sulfoxide (DMSO) (0.2% w/w) did not affect mucin secretion. Compounds increasing mucin secretion could alter the rate of mucociliary clearance and the mucus could provide a barrier to drug absorption. This could predispose patients to disease and affect the activity of delivered drugs, decreasing or increasing their clinical efficacy.

A comparison of three mucus-secreting airway cell lines (Calu-3, SPOC1 and UNCN3T) for use as biopharmaceutical models of the nose and lung

The aim of this work was to compare three existing mucus-secreting airway cell lines for use as models of the airways to study drug transport in the presence of mucus. Each cell line secreted mature, glycosylated mucins, evidenced by the enzyme-linked lectin assay. The secretagogue, adenylyl-imidodiphosphate, increased mucin secretion in SPOC1 (3.5-fold) and UNCN3T (1.5-fold) cells but not in Calu-3 cells. In a novel mucus-depleted (MD) model the amount of mucus in the non-depleted wells was 3-, 8- and 4-fold higher than in the mucus-depleted wells of the Calu-3, SPOC1 and UNCN3T cells respectively. The permeability of 'high mucus' cells to testosterone was significantly less in SPOC1 and UNCN3T cells (P < 0.05) but not Calu-3 cells. Mucin secretion and cytokine release were investigated as indicators of drug irritancy in the SPOC1 and UNCN3T cell lines. A number of inhaled drugs significantly increased mucin secretion at high concentrations and the release of IL-6 and IL-8 from SPOC1 or UNCN3T cells (P < 0.05). SPOC1 and UNCN3T cell lines are better able to model the effect of mucus on drug absorption than the Calu-3 cell line and are proposed for use in assessing drug-mucus interactions in inhaled drug and formulation development.

Evaluation of the toxicity of sodium dodecyl sulphate (SDS) in the MucilAir™ human airway model in vitro

The aim of the study was to use multiple in vitro assays to assess the effects of a model irritant, sodium dodecyl sulphate (SDS) (≤10 mM (0.29 %, w/v)), on an in vitro model of the airway, MucilAir™. The use of MucilAir™ in recovery studies was also explored. A 24 h exposure increased IL-8 release at an SDS concentration ≥0.63 mM (0.018 %, w/v). Mucin secretion increased and transepithelial electrical resistance (TEER) decreased at SDS concentrations ≥1.25 mM (0.04 %, w/v). Cytotoxicity (lactate dehydrogenase (LDH) release into basolateral chamber) was observed at SDS concentrations of ≥2.5 mM (0.07 %, w/v). The sensitivity of the assays was IL-8 release > TEER = mucin secretion > LDH release. After 7 days, full or partial recovery was observed for intermediate concentrations of SDS using all assays but not at 5 and 10 mM SDS. Morphologically, erosion and cell loss were observed at these concentrations. Resazurin metabolism at 7 days tended to decrease in a dose-dependent manner at SDS concentrations above 2.5 mM (0.07 %, w/v). Together, these data support a No Observable Effect Level of 0.31 mM (0.009 % w/v) SDS and the use of MucilAir™ as a relevant model for airway toxicity studies.

Biofilm formation and changes in bacterial cell surface hydrophobicity during growth in a CAPD model system

Peritonitis is a frequent complication of continuous ambulatory peritoneal dialysis (CAPD), with patients suffering recurrent attacks. The microorganisms most frequently implicated in the infection are the skin microflora, in particular, the coagulase-negative staphylococci such as Staphylococcus epidermidis. These microorganisms gain access to the peritoneal cavity via the in-dwelling silicone rubber catheter in the abdominal wall and often persist as biofilms on the surface of the catheter. The surface characteristics of S. epidermidis were monitored during growth in a CAPD in-vitro model together with their ability to adhere to silicone rubber substrata. Fresh dialysis fluid exerted an injurious effect on the cells leading to a decrease in cell numbers but during the simulated dialysis period the cells adapted to the applied stresses. Over a 96-h period in the model both a clinical isolate and a skin isolate of S. epidermidis adopted a more hydrophobic phenotype. The data presented here show that the bacteria grown in this in-vivo reflective CAPD model continually adapt to their environment and become more tolerant to the stresses imposed. The adapted cells were seen to colonise silicone rubber substrata.

Alcohol ethoxylates mediate their bacteriostatic effect by altering the cell membrane of Escherichia coli NCTC 8196

The minimum inhibitory concentration (MIC) of a homologous series of alcohol ethoxylates with the same head group size (E6) but differing in the number of carbon atoms in their 'tail group' from 10 to 16 was determined for Staphylococcus aureus NCTC 4163 and Escherichia coli NCTC 8196 using a turbidimetric assay. All the surfactants tested demonstrated bacteriostatic activity against both organisms. A tetrazolium assay showed that C14E6 and C16E6 had little effect on the membrane-bound dehydrogenase enzyme activity of E. coli NCTC 8196 compared with C10E6 and C12E6. C10E6 caused leakage both of K(+) and nucleotides in a concentration-dependent manner above its MIC of 0.2 mM. C12E6 caused some leakage at concentrations below its MIC (0.12 mM).

The human bronchial epithelial cell line 16HBE14o- as a model system of the airways for studying drug transport

The 16HBE14o- cell line, which forms polarised cell layers in vitro, provides a promising opportunity to develop a convenient epithelial cell culture model in which respiratory drug transport can be evaluated in vitro. This study investigated the effect of cell seeding density, collagen substratum and time in culture on the development of barrier properties in this cell line, after which the permeability of the 16HBE14o- cell layers to a series of solutes was studied. Seeding cells at a density of 2.5 x 10(5) cells per cm(2) on a monofibrillar Vitrogen-100 collagen substratum, followed by culture at an air-liquid interface for 6 days resulted in cell layers with a transepithelial electrical resistance (TER) of 247+/-47 omegacm(2) and an apparent permeability coefficient of 2.5 x 10(-6)cms(-1) for mannitol. The permeability of the 16HBE14o- cells to hydrophilic molecules (logP<1.9) was of an order of magnitude greater than that of typical alveolar cell cultures, possibly reflecting barrier properties more representative of the airways. More lipophilic drugs showed higher permeabilities indicating a sigmoidal relationship between permeability and lipophilicity similar to that observed for solute transport across primary cultured epithelial cell layers. These results indicate that under appropriate culture conditions, 16HBE14o- cell layers provide a discriminatory barrier to solute transport.

Toxicological evaluation of mixtures of nonionic surfactants, alone and in combination with oil

The toxicity to human bronchial (16-HBE14o-) epithelium cells of nonionic surfactants, polyoxyethylene-10-oleyl ether (C(18:1)E(10)), polyoxyethylene-10-dodecyl ether (C(12)E(10)), and N,N-dimethyl-dodecylamine-N-oxide (C(12)AO) alone or in combination with a range of pharmaceutically acceptable oils (namely, ethyl esters and triglyceride oils), was determined with the MTT (3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide) assay. Regardless of the presence of oil, all C(12)E(10)- and C(12)AO-containing systems were toxic at concentrations around or below their critical aggregation concentrations (as determined by surface tension measurements), suggesting that surfactant toxicity was due to the disruption caused by the partitioning of monomeric surfactant into the cell membrane. Systems prepared from C(18:1)E(10) alone or in combination with a low-molecular-weight oil, such as ethyl butyrate or tributyrin, were toxic above their critical aggregation concentration. In contrast, systems prepared from C(18:1)E(10) in combination with a high-molecular-volume oil (e.g., ethyl oleate, Miglyol 812, or soybean oil) were toxic only at concentrations significantly greater than their critical aggregation concentration, suggesting that in these cases surfactant toxicity was mediated by the aggregated form of the surfactant solubilizing components of the cell membrane. In the C(18:1)E(10)-stabilized system, it is proposed that toxicity was significantly reduced on incorporation of high-molecular-volume oils because these oils cause formation of a distinct oil core in the aggregates that leads to a reduction in the ability of the system to solubilize components of the cell membrane.

Formulation of inhaled medicines: effect of delivery vehicle on immortalized epithelial cells

The small volume of airway lining fluid renders it susceptible to alteration by the deposition of inhaled formulations. The increasing popularity of the pulmonary route for drug delivery has led to an increasing number of pharmaceutical excipients being incorporated into inhaled dosage forms. The effects of drug delivery vehicle on airway epithelial cells can be studied with the aid of cell culture models of the respiratory epithelium. The effects of pH, osmolarity, and lactose on epithelial cell layers were studied using 16HBE14o- cells. Mannitol flux was used to assess epithelial permeability, enzymatic conversion of 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) was used as a measure of epithelial cell metabolism, and release of the cytosolic enzyme lactate dehydrogenase was used as a measure of cell integrity. The effect of buffer composition on epithelial cell mucus secretion was studied using HT29-clone H cells, with mucus secretion measured using an enzyme-linked lectin assay. The permeability of 16HBE14o- cell layers was increased by apical fluid of pH 5, 6, and 9 as well as osmolarities of 96 and 545 mOsm. MTT conversion was reduced by apical fluid of pH 5 and 6 and osmolarity of 96 mOsm. Lactate dehydrogenase release was only increased by apical fluid of pH 9. No effect of lactose solution (100 mM) on the epithelial cells was observed. Mucus secretion by HT29-clone H cells was lowest in Dulbecco's modified Eagle medium (2.92 +/- 0.23 ng per cell layer) and was increased in phosphate-buffered saline with magnesium and calcium (4.28 +/- 0.38 ng per cell layer) and phosphate-buffered saline without magnesium and calcium (6.56 +/- 0.72 ng per cell layer). These results suggest that the physicochemical properties of inhaled formulations should be carefully controlled. The effect of buffer composition on mucus secretion suggests that even the application of "physiological" solutions may affect the epithelium. These cell models represent an opportunity to investigate the interaction of drug delivery vehicles with the epithelium.

Mitigation of surfactant erythrocyte toxicity by egg phosphatidylcholine

Polyoxyethylene alkyl ether surfactants have been shown to have excellent penetration enhancing abilities although they are associated with a high level of local toxicity. We have compared the toxicity of a range of polyoxyethylene alkyl ethers (Brij 96, Brij 76, Brij 56, 10 lauryl ether and 9 lauryl ether) to an anionic surfactant (sodium dodecyl sulphate (SDS)), an ampholytic surfactant (lysophosphatidylcholine) and a cationic surfactant (tetradecyltrimethylammonium bromide (TTAB)), in the presence and absence of egg phosphatidylcholine. The toxicity of the surfactants or phospholipid/surfactant mixtures was assessed by measuring haemolytic activity. The test samples were incubated with a suspension of red blood cells for 30 min and Drabkin's reagent was used to indicate the amount of haemoglobin released. All of the polyoxyethylene alkyl ethers, SDS, TTAB and lysophosphatidylcholine exhibited haemolytic activity at concentrations between 0.10 and 0.25 mM. The addition of egg phosphatidylcholine reduced the toxicity for all of the surfactants, with the toxicity of Brij 96 being mitigated to a greater extent than the toxicity of the other polyoxyethylene surfactants examined. The rate of haemolysis induced by Brij 96 or 10 lauryl ether was also reduced by increasing concentrations of phosphatidylcholine. As the phosphatidylcholine content of a mixed surfactant system comprising egg phosphatidylcholine: Brij 96 was replaced by lysophosphatidylcholine and fatty acid, the haemolytic action of the mixture increased markedly. The results from this study show that the toxicity of surfactants to erythrocytes can be mitigated by the addition of egg phosphatidylcholine. Synthetic surfactants combined with phosphatidylcholine may generate drug delivery systems worthy of more extensive investigation.

Light scattering investigations on dilute nonionic oil-in-water microemulsions

Dilute 3-component nonionic oil-in-water microemulsions formulated with either a polyoxyethylene surfactant (C18:1E10 or C12E10) or the alkylamine-N-oxide surfactant, DDAO (C12AO), and containing either a triglyceride or an ethyl ester oil have been examined using dynamic and static light-scattering techniques. Analysis of the results showed distinct differences in the tested oil's mode of incorporation into the microemulsion droplets, with both the molecular volume of the oil and the hydrophobic chain length of the surfactant being important. For example, microemulsions formulated by C18:1E10 and containing one of the larger molecular volume oils (that is, either a triglyceride, Miglyol 812, or soybean oil) or the ethyl ester of fatty acid oil, ethyl oleate, exhibited first a decrease and then an increase in hydrodynamic size and surfactant aggregation number, suggesting that the asymmetric C18:1E10 micelles became spherical upon the addition of a small amount of oil and grew thereafter because of further oil being incorporated into the core of the spherical microemulsion droplet. A similar conclusion of sphericity could not be drawn for microemulsions stabilized by C18:1E10 and containing one of the oils smaller in molecular volume (namely tributyrin, ethyl butyrate, or ethyl caprylate) where neither the aggregation number nor the hydrodynamic radius changed much upon the addition of oil. This result suggested that these oils were preferentially located in the interfacial surfactant monolayer, behaving in much the same way as a cosurfactant. A different trend of results, however, was seen for microemulsions prepared using C12E10 and C12AO, most likely because these surfactants produced approximately spherical micelles. In this case, the microemulsions containing the oils larger in molecular volume tended to exhibit an increase in surfactant aggregation number and hydrodynamic size, suggesting the growth of spherical micelles, while the smaller oils (in particular ethyl butyrate) caused a significant decrease in surfactant aggregation number incompatible with their being incorporated into the centre of the droplet, suggesting that the oils were being located in the interfacial surfactant monolayer. These results suggest that the various oils are incorporated into the microemulsions in very different ways.

Nonionic oil-in-water microemulsions: the effect of oil type on phase behaviour

The formation of oil-in-water (o/w) microemulsions stabilized by the nonionic surfactants, polyoxyethylene-10-dodecyl ether, polyoxyethylene-10-oleyl ether, N,N-dimethyldodecylamine-N-oxide and N,N-dimethyloleylamine-N-oxide and containing a variety of pharmaceutically acceptable oils, namely ethyl butyrate, ethyl caprylate, ethyl oleate and the triglycerides, soybean oil, Miglyol 812 and tributyrin, has been examined at 298 K. The effect on microemulsion formation of replacing water with phosphate buffered saline (PBS) and complete PBS has been established. In addition, the effect of changing temperature (from 298 to 310 K) on the phase behaviour of microemulsions formulated using PBS as continuous phase has been determined. Although some small differences in phase behaviour were noted when altering the continuous phase, the greatest difference in phase behaviour was observed when changing the experimental temperature, particularly for microemulsions stabilized by polyoxyethylene-10-oleyl ether. Regardless of the temperature and aqueous phase used, however the larger molecular volume oils (soybean oil, Miglyol 812 and ethyl oleate) were solubilized to a lower extent than the smaller molecular volume oils (namely, ethyl butyrate and ethyl caprylate). The only exception to this rule was when polyoxyethylene-10-oleyl ether was used as surfactant, particularly at 298 K, where it was the larger molecular volume oils that were solubilized to the greatest extent. Cloud point/phase inversion temperature experiments suggested that the higher molecular volume oils were incorporated into the microemulsions prepared using the polyoxyethylene-based surfactants in a different way than the smaller molecular volume oils and suggest that the smaller molecular volume oils are acting in much the same way as a cosurfactant in that they interchelate with their hydrophilic group interspersed in the surfactant head group region. As N,N-dimethyldodecylamine-N-oxide does not exhibit a cloud point it was not possible to determine the mode of oil incorporation in microemulsions prepared with this surfactant.

Use of alveolar cell monolayers of varying electrical resistance to measure pulmonary peptide transport

The apparent permeability coefficient (P(app)) of two fluorescently tagged model hydrophilic peptides, acXASNH(2) and acXAS(GAS)(7)NH(2), and (14)C-mannitol across monolayers of cultured rat alveolar epithelial cells of varying transepithelial electrical resistance (TER) has been examined. In line with their design features, the peptides were not degraded under the conditions of the test. Furthermore, no concentration dependence of transport of the tripeptide acXASNH(2) was observed over the concentration range studied, nor was any directional transport seen for either of the model peptides, indicating that under the conditions of the test they were not substrates for any transporters or efflux pumps. From the hydrophilic nature of the peptides (as assessed by their log P), and their inverse dependence of transport with molecular weight and TER, it was assumed that the peptides were transported across the cell monolayer passively via the paracellular route. The observed P(app) for the transport of (14)C-mannitol and the peptides across rat alveolar epithelial cell monolayers were found to be inversely (though not linearly) related to the measured TER and could be well-modeled assuming the presence of two populations of "pores" in the cell monolayer, namely, cylindrical pores of diameter 1.5 nm and large pores of diameter 20 nm. The relative populations of the two types of pores varied with the TER of the monolayer, with the number of large pores decreasing with an increase in TER (and the number of small pores taken as fixed). These results suggest that if the cell monolayer is well characterized with respect to the passage of a range of probe molecules across monolayers of varying electrical resistance, it should be possible to predict the P(app) of any hydrophilic peptide or drug crossing the membrane by the paracellular route at any desired TER using a monolayer of any electrical resistance, above a minimum value.

Systematic investigations of the influence of molecular structure on the transport of peptides across cultured alveolar cell monolayers

PURPOSE

To determine how the structures of peptides influence their alveolar permeability.

METHODS

The studies were performed using 14 synthetic 'model' peptides, labelled with a novel, non-intrusive amino acid fluorophore, and their transport studied using rat alveolar cell monolayers cultured on permeable supports.

RESULTS

The passage of the peptides across the epithelial cell monolayers is shown to be primarily paracellular, with an inverse dependence on molecular size, and an enhanced flux observed for cationic peptides. The apparent permeability coefficients (Papp) for the peptides (together with those for other organic solutes, taken from the literature) are shown to be well-modelled assuming two populations of 'pores' in the monolayers, modelled as cylindrical channels of radii 15 A and 22 nm. The former pores are shown to be numerically equitable with the monolayer tri-junctional complexes, and the latter are taken as monolayer defects.

CONCLUSIONS

The various monolayer Papp values correlate well with the results from in vivo transport experiments, and the conclusion is drawn that the pulmonary delivery of peptide drugs is perfectly exploitable.

Regulation of airway ciliary activity by Ca2+: simultaneous measurement of beat frequency and intracellular Ca2+

Airway ciliary activity is influenced by [Ca2+]i, but this mechanism is not fully understood. To investigate this relationship, ciliary activity and [Ca2+]i were measured simultaneously from airway epithelial ciliated cells. Ciliary beat frequency was determined, for each beat cycle, with phase-contrast optics and high-speed video imaging (at 240 images s-1) and correlated with [Ca2+]i determined, at the ciliary base, by fast imaging (30 images s-1) of fura-2 fluorescence. As a mechanically induced intercellular Ca2+ wave propagated through adjacent cells, [Ca2+]i was elevated from a baseline concentration of 45 to 100 nM, to a peak level of up to 650 nM. When the Ca2+ wave reached the ciliary base, the beat frequency rapidly increased, within a few beat cycles, from a basal rate of 6.4 to 11.6 Hz at 20-23 degrees C, and from 17.2 to 26.7 Hz at 37 degrees C. Changes in [Ca2+]i, above 350 nM, had no effect on the maximum beat frequency. We suggest that airway ciliary beat frequency is 1) controlled by a low range of [Ca2+]i acting directly at an axonemal site at the ciliary base and 2) that a maximum frequency is induced by a change in [Ca2+]i of approximately 250-300 nM.

Synthesis and properties of (6,7-dimethoxy-4-coumaryl)alanine: a fluorescent peptide label

The synthesis of racemic and l-(6,7-dimethoxy-4-coumaryl)alanine (Dmca) is described and some spectral and physicochemical properties are reported. The utility of Dmca as a highly sensitive and specific label for the quantitative detection of synthetic peptides in HPLC and in minimal essential media (MEM) is also described.

Stereoselective absorption and hydrolysis of cefuroxime axetil diastereomers using the Caco-2 cell monolayer model

Cefuroxime axetil, the orally active prodrug of cefuroxime is marketed as a 1:1 mixture of two diastereomers designated as R (1'R, 6R, 7R) and S (1'S, 6R, 7R). Prodrug hydrolysis is thought to occur during intestinal absorption, however little is known concerning the relative availability of cefuroxime from each isomeric form. The Caco-2 cell monolayer model was used to examine the possible stereoselectivity of absorption by measuring the accumulation and epithelial transport rate in the apical to basolateral direction of cefuroxime and cefuroxime axetil following application of the mixture (1.0 mM) or individual diastereomers (0.5 mM0 of cefuroxime axetil. Cefuroxime appearance in the basolateral chamber was in the order: mixture > R > S following application of the prodrug. The accumulation of unchanged cefuroxime axetil was S > R irrespective of the form applied, i.e. individual diastereomer or the mixture. Such stereoselective differences in both absorption and/or hydrolysis may contribute to the observed oral bioavailability (30-50%) of cefuroxime in vivo.

Control of the beat cycle of respiratory tract cilia by Ca2+ and cAMP

Beat frequency and the duration of the constituent recovery, effective, and rest phases of the beat cycle of respiratory tract cilia were measured photoelectronically before and after manipulation with ionomycin or isoproterenol. Both ionomycin, acting by increasing intracellular Ca2+, and isoproterenol, acting by elevating intracellular adenosine 3',5'-cyclic monophosphate (cAMP), increased beat frequency by reducing the duration of the three phases of the ciliary beat cycle in a similar manner. The addition of increasing concentrations of ATP to ciliated cells permeabilized by exposure to saponin caused a pattern of phase reduction indistinguishable from that observed in whole cells. The beat frequency of permeabilized cells was slower than that of whole cells and insensitive to changes in Ca2+ and cAMP. Ca2+ and cAMP may regulate ciliary beat frequency by acting at a common site within intact cells, possibly regulating the rate at which the axoneme can use ATP or the availability of ATP to the axoneme.