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.