CELL-ATTACHED MEASUREMENTS OF ATTOFARAD CAPACITANCE STEPS IN RAT MELANOTROPHS

Capacitance changes in cell-attached patches of rat melanotrophs were measured by a high-frequency lock-in amplifier. The background noise o f around 30 aF allowed the detection of discrete steps due to fission (endocytosis) and fusion (exocytosis) of vesicles with diameters as sm all as 60 nm. The amplitude of both types of steps was similar with a modal value of around 300 aE The frequency of these steps was not chan ged, if secretagougues such as ionomycin and/or dibutyril cAMP, were a pplied to the bathing solution. Moreover, this treatment did not resul t in an increased appearance of expected 2000 to 3000 aF steps due to exocytosis of secretory granules. We conclude that the likely explanat ion for recorded capacitance steps is that they represent the constitu tive vesicle traffic. From the typical frequency and amplitude of thes e events (around 1 min(-1), 300 aF) in a membrane patch (26 fF) it is estimated that the whole membrane of a rat melanotroph may be ingested under our conditions in 1 to 2 h.