Author/Editor		Poberaj, Igor; Rupnik, Marjan; Kreft, Marko; Sikdar, Sujit K; Zorec, Robert
Title		Modeling excess retrieval in rat melanotroph membrane capacitance records
Type		članek
Source		Biophys J
Vol. and No.		Letnik 82, št. 1
Publication year		2002
Volume		str. 226-32
Language		eng
Abstract		We have used the patch-clamp technique to monitor changes in membrane capacitance (Cm) elicited by fast and spatially homogenous rises in cytosolic calcium concentration ((Ca2+)i) using flash photolysis of NP-EGTA. Average peak (Ca2+)i aplitudes of 20-25 micro M triggered threee different types of responses in Cm:(i); In 41% of cells, a rise in (Ca2+)i activated a monotonic increase in Cm followed by a slow decline to resting values; (ii) In 30% of cells, the rise in Cm was clearly characterized by two dynamic components, consisting of a rapid and a slow-endocytosis cycle; (iii) In 28% of cells, after the initial rapid rise in Cm, endocytosis exhibited excess retrieval that was characterized by a decline in Cm below resting Cm. 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 indipendent dynamic components; and model C, a model in which, in addition to 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 Cm recorded in rat melanotrophs. The association of excess retrieval exclusiverly 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 flow that might have occured as the fusion pore opened permanently.
Descriptors		PITUITARY GLAND EXOCYTOSIS ENDOCYTOSIS CALCIUM CYTOSOL PATCH-CLAMP TECHNIQUES CELL MEMBRANE RATS MODELS, THEORETICAL