We have shown previously that insulin stimulates fluid phase endocytosis in 3T3-Ll adipocytes (Gibbs et al., 1986). Using ['4Clsucrose as an endocytotic marker, we show here that phenylarsine oxide, a trivalent arsenical which binds neighboring dithiols, blocked not only insulin-stimulated fluid phase endocytosis, but basal endocytosis as well. The K, for this process was 6 pM in the presence or absence of insukin and the time required for inhibition was less than 2.5 min, the limit of detection in our assay system. These results can be compared with the inhibitory effect of phenylarsine oxide on insulin-stimulated glucose transport. Although the Ki for insulin-stimulated transport (7 pM) was similar to that for inhibition of endocytosis, basal glucose transport was not affected by the inhibitor. Further, when cells were prestimulated with insulin causing maximal stimulation of the glucose transport rate, phenylarsine oxide induced a time-dependent reduction to the basal rate (t,,, of 10 min), despite the fact that endocytosis was blocked immediately. This observation suggests that if the transporter is recycled by an exocytotic/endocytotic mechanism, it is distinct from fluid-phase endocytosis/exocytosis, which is a vesicle-mediated process, and provides further evidence that the transporter may undergo intrinsic activationiinactivation which does not require vesicle movement.
'Intrinsic activation is used here to mean that a population of nonfunctional (or partially functional) transporters exists, which undergoes acLivation in the presence of insulin independent of translocation.