1. The excitatory actions of phenylephrine (Phe) and acetylcholine (ACh) on the smooth muscle of rabbit portal vein were investigated and compared by using whole-cell and single channel configurations of the patch clamp technique, in combination with a modified concentration jump method. 2. At negative holding potentials with KCl (0.1 mm EGTA) electrodes, rapid applications of Phe (> 1 mum) and ACh (> 10 mum) both resulted in biphasic responses consisting of fast outward or inward currents and a long-lasting inward current with an increased noise level. 3. The slow inward current was still recorded when the cell was dialysed with caesium aspartate solution complemented with 10 mm EGTA, in order to eliminate contributions of calcium-dependent conductances (K+ and Cl- currents). Phe was more potent at activating the current than ACh. ion replacement experiments revealed that both the Phe- and ACh-induced slow inward currents are cation-selective conductances (CS currents). 4. The I-V relationships of the Phe- or ACh-induced CS currents were similar both for the instantaneous peak and the steady state. The tail current analysis over a wide range of membrane potentials (- 150 to + 100 mV) showed that depolarizations to very positive potentials (> + 50 mV) from near the resting membrane potential (-40 mV) can produce a several-fold increase in the steady-state activation of the CS currents, but the maximal activations were in most cases not observed even at + 100 mV. 5. Externally applied Cd 2+ produced a quick and reversible inhibition of both the Phe- and ACh-induced CS currents. This inhibition seemed almost voltage independent and the concentrations of half-inhibition were 100 and 129 mum for Phe and ACh, respectively. 6. Single channel activities were recorded in the presence of Phe or ACh using the outside-out membrane patches. The unitary conductances and reversal potentials of the Phe- and ACh-activated channels were 23 pS, + 5.2 mV and 25 pS, + 4.1 mV, respectively, and the open lifetimes evaluated for 50 mum Phe and 500 mum ACh were of a similar order (the longer open times at - 60 mV for Phe and ACh were 4.3 and 4.4 ms, respectively). In addition. the relative open probability (P(o, rel) was no more than 0.2 in the voltage range of - 100 to - 30 mV for both 50 muM Phe and 500 muM ACh, suggesting low open probabilities of the Phe and ACh-activated CS channels near the resting membrane potential. 7. The activation of the CS current by 100 mum Phe was completely inhibited by application of 10 mum prazosin but not affected by that of 1 mum atropine, while the opposite results were observed for the CS currents activated by 100 mum ACh. 8. From these results, we concluded that both the alpha1-adrenergic and muscarinic receptor regulate a common CS channel, and this may in part be responsible for the excitatory cholinergic and adrenergic actions in the rabbit portal vein.
