Characteristics of a Low Affinity Passive Ca²⁺ Influx Component in Rat Parotid Gland Basolateral Plasma Membrane Vesicles

Abstract.

We have previously reported the presence of two Ca²⁺ influx components with relatively high (K_Ca= 152 ± 79 μm) and low (K_Ca= 2.4 ± 0.9 mm) affinities for Ca²⁺ in internal Ca²⁺ pool-depleted rat parotid acinar cells [Chauthaiwale et al. (1996) Pfluegers Arch. 432:105–111]. We have also reported the presence of a high affinity Ca²⁺ influx component with K_Ca= 279 ± 43 μm in rat parotid gland basolateral plasma membrane vesicles (BLMV). [Lockwich, Kim & Ambudkar (1994) J. Membrane Biol. 141:289–296]. The present studies show that a low affinity Ca²⁺ influx component is also present in BLMV with K_Ca= 2.3 ± 0.41 mm (V_max= 16.36 ± 4.11 nmoles of Ca²⁺/mg protein/min). Our data demonstrate that this low affinity component is similar to the low affinity Ca²⁺ influx component that is activated by internal Ca²⁺ store depletion in dispersed parotid gland acini by the following criteria: (i) similar K_Ca for calcium flux, (ii) similar IC₅₀ for inhibition by Ni²⁺ and Zn²⁺; (iii) increase in K_Ca at high external K⁺, (iv) similar effects of external pH. The high affinity Ca²⁺ influx in cells is different from the low affinity Ca²⁺ influx component cells in its sensitivity to pH, KCl, Zn²⁺ and Ni²⁺. The low and high affinity Ca²⁺ influx components in BLMV can also be distinguished from each other based on the effects of Zn²⁺, Ni²⁺, KCl, and dicyclohexylcarbodiimide. In aggregate, these data demonstrate the presence of a low affinity passive Ca²⁺ influx pathway in BLMV which displays characteristics similar to the low affinity Ca²⁺ influx component detected in parotid acinar cells following internal Ca²⁺ store depletion.