Pressure threshold (Pth) and suprathreshold pressure sensitivity (Sth) are important measures of the pressure-discharge characteristics of arterial baroreceptors. An in vitro preparation of the rat aortic arch-aortic nerve has been used to assess the influence of extracellular ion concentration, distensibility, smooth muscle activation and rapid resetting on single fibre baroreceptor discharge. 2. Changes in extracellular cations alter Pth and Sth in a reciprocal manner, suggesting that these two properties share common excitatory mechanisms probably at the level of membrane ion conductance channels. 3. During normal development and ageing in normotensive rats, Pth and blood pressure are fairly constant even during periods of greatly changing aortic distensibility. Sth increases progressively to maturity and then decreases somewhat with advanced age. 4. During hypertension, changes in distensibility in spontaneously hypertensive rats do not account for changes in Pth and Sth. 5. The capacity of arterial baroreceptors to rapidly reset during acute changes in the conditioning mean arterial pressure is not altered by chronic resetting, decreases in distensibility or by differences in the initial Pth or Sth of individual baroreceptors. 6. Within the maximal physiological or pathophysiological range, the prevailing or conditioning mean arterial pressure appears to be the most potent modulator of arterial baroreceptor discharge.
