The superficial buffer barrier hypothesis in venous smooth muscle

The interaction of Ca$\sp{2+}$ transport in the plasmalemma and the sarcoplasmic reticulum (SR) was investigated in smooth muscle of the rabbit inferior vena cava. SR depletion prior to stimulation of Ca$\sp{2+}$ influx induced a delay of 30-70 seconds between the intracellular Ca$\sp{2+}$ signal and force development. This delay was abolished by the application of caffeine. These data support the Superficial Buffer Barrier hypothesis, which holds that SR functions as a buffer barrier against Ca$\sp{2+}$ entry into the central cytoplasm.Thapsigargin completely blocked refilling of the SR regardless of Ca$\sp{2+}$ entry pathways, indicating that the SR Ca$\sp{2+}$-ATPase is essential for this process.Caffeine, ryanodine and thapsigargin, which inhibit SR Ca$\sp{2+}$ accumulation, increased the steady state intracellular Ca$\sp{2+}$ concentration ((Ca$\sp{2+}\rbrack\sb{\rm i}$). Measurements of Mn$\sp{2+}$ induced quench of the intracellular fura-2 signal during pharmacological manipulation of the SR content showed that these three agents did not stimulate divalent cation entry. On the other hand, stimulation with noradrenaline caused a marked increase in Mn$\sp{2+}$ influx, which was blocked by 2 mM Ni$\sp{2+}$. Mn$\sp{2+}$ entry stimulated by high K$\sp+$ solution was blocked by 1 $\mu$M diltiazem.After first raising (Ca$\sp{2+}\rbrack\sb{\rm i}$ with a high K$\sp+$, high Ca$\sp{2+}$ solution, subsequent replacement with a Ca$\sp{2+}$-free external solution will result in a decline in (Ca$\sp{2+}\rbrack\sb{\rm i}$. Comparison between the (Ca$\sp{2+}\rbrack\sb{\rm i}$ decays and their converted rate-concentration curves under different conditions shows that prior inhibition of SR Ca$\sp{2+}$ accumulation by caffeine or thapsigargin actually slows the rate of decline in (Ca$\sp{2+}\rbrack\sb{\rm i}$ which mostly reflects the process of Ca$\sp{2+}$ extrusion from the cells. There were no significant additive effects between the inhibition of Na$\sp+$/Ca$\sp{2+}$ exchanger and that of SR Ca$\sp{2+}$ accumulation. It is concluded that SR dysfunction has an inhibitory effect on Ca$\sp{2+}$ extrusion which results in an increase in the steady state (Ca$\sp{2+}\rbrack\sb{\rm i}$. The above results support the component of the Superficial Buffer Barrier hypothesis which proposes that ER Ca$\sp{2+}$ is discharged to the extracellular space via vectorial Ca$\sp{2+}$ release from the SR lumen