Whole body retention in rats of different 191Pt compounds following inhalation exposure.

The whole body retention, excretion, lung clearance, distribution, and concentration of 191Pt in other tissues was determined in rats following a single inhalation exposure to different chemical forms of 191Pt. The chemical forms of 191Pt used in study were 191PtCl4, 191Pt(SO4)2, 191PtO, and 191Pt metal. Immediately after exposure most of the 191Pt was found in the gastrointestinal and respiratory tract. Movement of the 191Pt through the gastrointestinal tract was rapid, most of the 191Pt being eliminated within 24 hr after exposure. Lung clearance was much slower, with a clearance half-time of about 8 days. In addition to the lungs, kidney and bone contained the highest concentrations of 191Pt.


Introduction
Automotive manufacturers have incorporated into the exhaust train a noble metal oxidation catalyst which reduces the concentrations of carbon monoxide (CO) and total hydrocarbons (THC) in the exhaust. The substrate material used in the catalyst is coated with a small amount of platinum (Pt) and palladium (Pd) (approximately g Pt and 0.5 g Pd). It is not unreasonable to assume that some of the catalytic material will be emitted into the atmosphere, and recent studies at this laboratory have identified the presence of noble metals in the exhaust. The chemical form of the Pt and Pd emitted in the exhaust has not been determined. We have given first consideration to the study of Pt compounds because of the larger quantity of this metal used in the catalyst (1).
The toxicity of Pt is variable, depending on its chemical form. Investigators have described the toxicity of the soluble Pt salts following industrial exposure and use the term platinosis in referring to the respiratory and/or cutaneous reactions (2)(3)(4). *U.S. Environmental Protection Agency, National Environmental Research Center, Environmental Toxicology Research Laboratory, Cincinnati, Ohio 45268.
The threshold limit value for industrial exposure to the soluble Pt salts is 2 ,ug/m3 as Pt (5). With the emission of Pt in automobile exhaust, the lungs could serve as the major portal of entry for human exposure. Because of the paucity of available information, this study was undertaken to provide data on whole body retention, tissue distribution and excretion of different chemical forms of Pt following inhalation exposure. Major emphasis was placed on the fate of PtO and Pt metal because it was believed that these chemical forms might be formed preferentially in the high temperature of the exhaust.

Animals
Eighty-seven outbred albino rats (Charles River CD1 strain) weighting 100-120 g were used for exposure to each l9lPt compound. The animals were maintained on a commercial diet (Purina Lab Chow) and tap water ad libitum.

Aerosol Exposure
The animal exposure chamber was constructed according to the design of Raabe et alt (6). The design allows for the simultaneous exposure of the nose only; each of the 87 rats breathes its own portion of the same aerosol during the exposure period.
A Retec X 70/N nebulizer was operated at 15 psig pressure to produce the aerosol of aqueous droplets. The 2 1./min (STP) of air flowing into the nebulizer was diluted by 7 1./min of dry air at the entrance into the drying tube. Thermo-Systems, Inc. Model 301Z Aerosol Neutralizer served the dual function of neutralizing the particle electrostatic charge as well as providing the capacity for removing the water from the aqueous droplets by the addition of heat. A heating tape of 300-W capacity controlled by a rheostat was wrapped around the Neutralizer, and the temperature within the Neutralizer was elevated 20-25°C as an aid in drying the particulate. The aerosol was further diluted at the entrance to the trapezoidal plenum so that the total air flow through the exposure chamber was 80 1./min. Samples were taken during each exposure for analysis. Pt(SO4)2 in a slightly acidified aqueous solution to provide a final 10 ml volume of approximately 1.5 mg/ml. Following passage of the aerosol through the drying tube, the dry particles of the solute were then passed through a Lindberg furnace tube (600°C) to decompose the Pt(SO4)2 into PtO2 and SO2. The S02 and the H2SO4 vapor formed by volatilization of the original acidified solution were largely removed by reaction with the CaCO3 in an absorption tube, so that only the PtO2 remained in the aerosol stream entering the exposure chamber. The final concentration of PtO2 was 7-8 mg/m3. Pt Metal Particulate: For the Pt metal exposure, a 10.0 ml supply of 191,193mPtCl4, assaying 6.85 mCi for l9lPt in 0.5M HCI was used. To it was added 21.9 mg of stable PtCl4 to provide a final concentration of 2.2 mg/ml. The PtCl4 aerosol was decomposed in the furnace to Pt metal particles and chlorine gas. The chlorine and the HCI vapor volatilized from the original acidified solution were absorbed by activated char surrounding the tubular space in the absorption tube section. Air flows were similar to those used in the other exposures and calculated to give a final concentration of 7-8 mg Pt metal/m3 in the exposure chamber atmosphere.

Radioactive Counting
Following exposure, the rats were washed to minimize radioactive contamination of the skin and hair. Whole body counts were then made on all animals used in the retention studies. The animals were counted daily for the first few days and then every other day for the duration of the experiment. The rats were housed in metabolism cages, and 24hr urine and feces samples were collected for counting. A 200-channel gamma spectrometer with a 5in. Nal (TC) crystal was used for whole body counts. Some of the animals were sacrificed and dissected so that the lung, gastrointestinal tract (contents not removed), kidney, bone, and liver burdens of l9lPt could be determined by tissue counting. Tissue, urine, and feces samples were counted in a refrigerated well-type scintillation spectrometer.
The radioactive platinum solution consisted of a mixture of 191,193Pt, at least 50% being l9lPt. Only the l9lPt was counted, and all values were corrected for decay (191 Pt has a half-life of 3 days).

Results
There was considerable individual variation in the amount of radioactivity present in some of the animals immediately after exposure. Although differences in the total volume of air breathed by individual animals may have been partly responsible, the major reason for the variation was attributed to the rat removing its nose from the exposure port and trying to turn in the cylinder.
Whole body retention curves for l9lPt as a function of the type of compound and time following inhalation exposure are presented graphically in Figures 1 and 2 Excretion Radioactive counts of 24-hr urine and feces samples from rats following inhalation exposure to the different l9lPt compounds indicated that most of the l9lPt was eliminated in the feces, and only a small amount was excreted in the urine (Figs. 3 and  4). These values support the whole body data that showed that only a small amount of l9lPt was retained by the animals. Most of the l9lPt was rapidly cleared from the lungs (probably by mucociliary action), swallowed, and excreted via the feces. The presence of 19lPt in the urine indicated the absorption of a small fraction of the 19lPt, although it was impossible to determine the relative contributions of lung and of gastrointestinal absorption to the total body burden.
Tissue Clearance  followed by a slower clearance phase during the remainder of the post-exposure period. No significant' difference in whole body retention of l9lPt was apparent following inhalation exposure to the different chemical forms; however, clearance of 191PtCl4 seemed to be more rapid.   Figures 5 and 6. Immediately after exposure most of the radioactivity was found in the gastrointestinal and respiratory tract. Movement of the l9lPt through the gastrointestinal tract was rapid, most of the l9lPt being eliminated within 24 hr after exposure. Removal of the deposited l9lPt from the lungs was much slower. The initial lung burdens for 19'Pt metal had a coefficient of variation of about 34% and a range of about a factor of 2.6. The initial lung burdens for l9lPt metal represented about 14% of the initial body burdens. For the l9lPtO exposure, the initial lung burdens had a coefficient of variation of 30% and a range of about 3. The initial lung burdens for l9lPtO were about 16% of the initial body burdens. This spread was consistent with the reported variability in rodent inhalation exposures (6,7). The percentage of the initial lung burden retained for the duration of the study is given in Table 1.
The data indicate that Pt(S04)2 may be more rapidly mobilized from the lung.
The clearance of 19lPt from the lungs may be divided into an initial rapid decrease of activity  over the first 24 hr followed by a slower clearance phase during the rest of the post-exposure period.
For the slow phase, the clearance half-time was about 8 days.
The distribution and concentration of 191Pt in other tissues following exposure to 191Pt metal are given in Table 2. In addition to the lung and trachea, the kidney and bone contained the highest amount of 191Pt. The relative large amount of 191Pt found in the kidney and bone suggests that these organs accumulate this element. The low radioactive count for the brain suggested the possibility that the circulating 191Pt may be complexed to large molecules that do not cross the blood brain barrier.

Discussion
Clinical platinosis has been found in individuals working with the complex platinum salts (sodium, potassium, or ammonium tetrachlorplatinate or hexachloroplatinate) encountered in refining and analysis of the metal. The majority of individuals working in atmospheres containing these salts develop respiratory symptoms or skin lesions or both only after occupational exposure periods of months or years. Only one case of dermatitis has been reported from metallic platinum (8). Evidence indicates that the clinical syndrome of platinosis in man is the result of hypersensitivity to platinum salts. The platinum salts apparently act as a hapten and combine with an endogenous protein to form an allergenic hapten-protein complex (9).
The results from this study indicate that all the chemical forms tested [PtCl4, Pt(SO4)2, PtO, Pt] were absorbed from the lungs and entered various body pools. Also a comparison of absorption following inhalation and oral exposure indicate that the respiratory route is a more effective portal of entry for Pt (10). It is reasonable to assume that these forms of Pt are also absorbed following inhalation exposure in humans. The question remains why metallic and other simple salts of Pt have not been reported as being effective agents in producing platinosis. Platinosis is a rare disease outside platinum refineries; however, if platinum increases as an atmospheric pollutant, one might anticipate concomitant increase in allergic reactions.