Type I cells of carotid body from rats treated with 5-OH-dopa and L-dopa: an electron microscopical study.

The ultrastructure of the Type I cells in rat carotid bodies was studied after treatment with 5-OH-Dopa and L-Dopa. Type I cells from the L-Dopa treated rats were also analysed by morphometric methods. After 5-OH-Dopa treatment, the majority of dense-cored vesicles of the Type I cells were almost completely filled with very electron-dense material. The vesicles were mainly distributed close to the plasma membrane. After L-Dopa treatment, the ultrastructure of the Type I cells resembled that after 5-OH-Dopa treatment, except that the increase in electron density of the vesicle content was less pronounced. The morphometric analysis revealed, as previously described for normal carotid bodies, two subclasses of Type I cells, small vesicle cells (SVC) and large vesicle cells (LVC). Compared to those of normal rats, the mean profile diameter of the vesicles in the L-Dopa treated rat carotid bodies were enlarged (for SVC from 47 to 55 nm and for LVC from 63 to 76-78 nm). Furthermore, the volume density of vesicles in both SVC and LVS was increased. The mean volume of the SVC was markedly increased after L-Dopa treatment for 60 h. These observations indicate that the Type I cells possess mechanisms for uptake of amine precursors and synthesis of monoamines. Though the vesicle may be involved in an 'endocrine-like' function, they may also be related to the chemoreceptor function of the carotid body.