, Volume 18, Issue 1, pp 61-80

The anomalous rectification and cation selectivity of the membrane of a starfish egg cell

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The cation selectivity and its relation to the inward-going rectification of the immature egg cell membrane of a starfish,Nordora punctiformis, were studied and the following results were obtained. (1) When the external saline contains usual ion species the cell membrane at rest is predominantly permeable to K ions. The K chord conductanceg k depends on the electrochemical potential of K ions, ΔV=V−E K′ and the external K concentration [K+] o by $$g_4 = A\left[ {1 + \exp \left( {\frac{{\Delta V - \Delta V_h }}{\upsilon }} \right)} \right]^{ - 1} ([K^ + ]_0 )^{\tfrac{1}{2}} $$ \((\Delta V_h \underset{\raise0.3em\hbox{\(\smash{\scriptscriptstyle\cdot}\)}}{ \doteq } - 15mV,\upsilon \underset{\raise0.3em\hbox{\(\smash{\scriptscriptstyle\cdot}\)}}{ \doteq } 7mV)\) . (2) The permeability sequence of monovalent cations and the permeability ratiosP x/PK of the cell membrane at rest obtained with membrane tpotential measurements are, T1 (1.5)>K(1.0)>Rb(0.3 to 0.4)>NH4(0.03 to 0.04)>Na, Cs. (3) Current-voltage relations obtained when the external solution contains Rb+, Cs+ or T1+ in addition to K+ show: (a) Rb+ and Cs+ decrease the K conductance and the rate of decrease becomes greater with an increasing hyperpolarization, thereby the inward-going rectification is reduced; (b) the membrane conductance shows an ‘anomalous mole fraction dependence’ in Tl-K media, i.e., the conductance becomes minimum at a certain mole fraction; and (c) the current-voltage relation often shows a transitiontype behavior suggesting that the membrane undergoes metastable states during an ncrease of hyperpolarization.