Modeling excess retrieval in rat melanotroph membrane capacitance records.

We have used the patch-clamp technique to monitor changes in membrane capacitance (C(m)) elicited by fast and spatially homogeneous rises in cytosolic calcium concentration (Ca(2+)) using flash photolysis of NP-EGTA. Average peak Ca(2+) amplitudes of 20-25 microM triggered three different types of responses in C(m): (i) In 42% of cells, a rise in Ca(2+) activated a monotonic increase in C(m) followed by a slow decline to resting values; (ii) In 30% of cells, the rise in C(m) was clearly characterized by two dynamic components, consisting of a rapid and a slow exo-endocytosis cycle; (iii) In 28% of cells, after the initial rapid rise in C(m), endocytosis exhibited excess retrieval that was characterized by a decline in C(m) below resting C(m). The aim of this work is to develop a unified mathematical model with a minimum number of parameters that would describe all the observed types of responses. Three models were considered: Model A, a model with a single component of exo-endocytosis cycle; model B, a model consisting of a sum of two independent dynamic components; and model C, a model in which, in addition to the two dynamic components as in model B, excess retrieval due to a lipid flow through the reversal closing of the fusion pore during the rapid component of exo-endocytosis cycle was considered. The results show that the latter model describes all the types of responses in C(m) recorded in rat melanotrophs. The association of excess retrieval exclusively with the rapid, but not the slow, exocytosis indicates that some fusing vesicles mediate a lipidic flux during the reversal closing of the fusion pore, whereas those entering the slow phase of exocytosis may fuse with the plasma membrane completely and are retrieved by other endocytic machinery, independent of the lipid flow that might have occurred as the fusion pore opened permanently.