Inward movement of ca²⁺ in k⁺-depolarized rat vascular smooth muscle

The kinetics of the Ca²⁺ (⁴⁵Ca²⁺) uptake in segments of rat aorta and its relationship with tension development in the presence of K⁺ concentrations ([K⁺]o) ranging from 3.5 to 100 mm was investigated. It was found that : 1) The curve relating ⁴⁵Ca²⁺ uptake and time was virtually linear during the first 3 min. Therefore, the influx calculated as uptake in 3 min/3 min was not significantly different from that calculated as the slope of the uptake curve. The actual uptake of the cation during the loading period would be underestimated by about 30 percent if the loss of Ca²⁺ from that compartment during a 2 h clearing of the extracellular space in La³⁺ medium, is ignored. 3) ⁴⁵Ca²⁺ activity remaining in the tissue after a 3 min labelling period followed by a 2 h washout can reliably be corrected to calculate Ca²⁺ influx. 4) The K⁺-induced tension development showed no significant correlation with the total exchangeable Ca²⁺ while it appeared as a steep function of Ca²⁺ influx between 15 and 80 mm [K⁺]o. 5) In [K⁺]o = 100 mm Ca²⁺ influx was 5 to 6 times larger than in normal [K⁺]o (5.3 mm). The estimated Ca²⁺ permeability (PCa) increased linearly as a function of [K⁺]o (5.3 to 100 mM). In [K⁺]o = 100 mm, PCa was estimated to be about 17-fold that in [K⁺]o = 5.3 mM. Similarly, the increased in Ca²⁺ conductance (GCa) for the same change in [K⁺]o was estimated to be 14-fold.