Changes in spontaneous contractile activity of smooth muscles in the gastrointestinal tract of rats under long-term intake of a mixture of polypropylene nanoparticles and microparticles

Background. Plastic is the most common pollutant in the environment. In natural conditions, plastic undergoes mechanical and photochemical degradation, forming micro- and nanoparticles that are accumulated by living organisms. Regular intake of plastic micro- and nanoparticles (MNP) in the gastrointestinal tract (GIT) leads to the development of inflammatory processes in the alimentary canal walls and disrupts intestinal microbiota. However, the contractile activity of smooth muscles in the GIT under these conditions is yet to be studied. The aim of the study was to investigate the spontaneous contractile activity of circular smooth muscles of the antrum and caecum of rats under long-term peroral intake of MNP of one of the most common plastic pollutants for the environment, polypropylene (PP). Materials and Methods. The suspension of polypropylene particles was prepared using disposable dishes by Roursgaard’s method with slight modifications. The determination of the average hydrodynamic diameter of PP particles in the aqueous suspension involved the method of dynamic light scattering. The animals in the experimental group received the PP suspension in their drinking water (at a daily dose of 2.5 mg/kg) for 6 weeks. The tenzometric experiments were conducted in the isometric mode of registration, using isolated circular smooth muscle preparations from the antrum and caecum of rats. The spontaneous contractions were analyzed by mechanokinetic analysis with the estimation of force, time, velocity, and impulse parameters. Results. The polypropylene suspension contained particles of up to 4 µm, 34.1 % of which were < 1 µm. Long-term peroral intake of PP into the organism was accompanied by the impairment of the spontaneous contractile activity of smooth muscles in the antrum and caecum: a decrease in the frequency and modulation of mechanokinetic parameters of some contractions. In case of the antrum muscles, there was an increase in force and velocity parameters, time parameters were below the control ones, while the impulse parameters remained unchanged. Under the effect of PP, the mechanokinetic parameters of contractions in the caecum demonstrated the following changes: there was a considerable increase in force parameters for the amplitude and phase of contraction, as well as all the time and impulse parameters, whereas velocity parameters were considerably decreased. Conclusions. Prolonged intake of polypropylene MNP into the organism causes changes in the frequency of spontaneous smooth muscles contractions, likely due to impaired functioning of pacemaker cells; whereas changes in amplitude parameters are likely due to MNP’s action on smooth muscle cells.