Characteristics of a low affinity passive Ca2+ influx component in rat parotid gland basolateral plasma membrane vesicles

We have previously reported the presence of two Ca2+ influx components with relatively high (K-Ca = 152 +/- 79 mu M) 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 mu M 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.