Abstract
In order to elucidate the glucoreceptor mechanism involved in insulin release and the character of insulin release under various cationic environments, we have studied the dynamics of insulin release from the rat pancreas perfused with K+, Rb+ and Li+ media (6.2, 24.8 mM) in the presence of glucose and in its absence. The results are as follows.
1. Perfusion in K+, Li-free medium and, 6.2 mM K+, Rb+ or Li+ medium: Insulin release in the second phase occurred in K+, Lit-free, and 6.2 mM Li+ media without glucose, but infusion of 6.2 mM K+ or Rb+ medium did not induce insulin release. Insulin response to infusion of K+, Li-free medium containing 10 mM theophylline was 3 times as large as that of the control in the absence of glucose, but 10 mM theophylline did not affect insulin release in a 6.2 mM Li+ medium. Analysis of Hill plots of rates of insulin release induced with glucose in various media showed that Km, n (Hill constant) and log K (equilibrium constant between glucoreceptor and glucose) were 6.5mM, 1.4, and -1.1 (K+, Li-free), 5.4 mM, 1.4, and -1.0 (6.2 mM Lit), 8.9 mM, 2.4, and -2.3 (6.2mM K+), and 8.7 mM, 2.3, and -2.1 (6.2 mM Rb+), respectively.
2. Perfusion in 24.8 mM K+, Rb+ or Li+ medium: Infusion of 24.8 mM K+ or Rb+ medium provoked a transient insulin release, but that of 24.8 mM Li+ medium did not induce insulin release. The amount of insulin release due to 24.8 mM K+ stimulation following preperfusion with a 12.4mM Li+ plus 6.2 mM K+ medium decreased to 47% of that of the control.
3. Insulin responses due to 16.7 mM α-and β-D-glucose in 6.2 mM K, Rb+ and Li+ media switched from 6.2 mM K+ medium: The average rate of insulin release and the maximal values were 76+10μU/ml/min and 127±30 itU/ml (the α anomer), and (74±8μU/mlmin) and 120±13μU /ml (the β anomer) in a 6.2 mM K+ medium, 34±14μU/ml/min and 57±18μU/ml (the a anomer), and 33μ±10μU/ml and 69±23 tiU/ml at 6 min (the β anomer), in a 6.2 mM Rb+ medium, 41±13μU/ml/min and 72±19μU/ml (the a anomer), and 121±29μU/ml/min and 193±49μU/ml (the β anomer), respectively.
We conclude that Li+ increased in affinity of the glucoreceptor for glucose to cause more enhanced insulin release than those in 6.2 mM K+ or Rb+ medium, and diminished insulin release as compared to that in a K+, Li-free medium through stabilization of the B cell plasma membrane, and moreover provided a greater stimulation of the β anomer than of the a anomer through a conformational change of the glucoreceptor.
