The effect of external Ca2+ concentration on the contractility of bovine mesenteric lymphatics

Spontaneous isometric contractions of isolated lymphatic vessels were studied in the single sucrose gap apparatus. Contractions were also elicited by applying single electrical pulses either within the Krebs solution (field stimulation) or across the sucrose gap (gap stimulation), or by means of high-K+ Krebs. Reductions in the calcium concentration increased the frequency but decreased the force of spontaneous contractions and ultimately abolished them. The loss of mechanical activity in calcium free solution was associated with failure of action potential firing. Increasing external [Ca2+] had no apparent effect on the force of spontaneous contractions but these became less frequent and finally stopped. The calcium antagonist D600 decreased the force of spontaneous contractions without increasing their frequency. The contractions elicited by electrical stimulation and by high-K+ Krebs were less readily abolished by calcium removal. This was particularly true of field stimulation where contractions could still be elicited up to 40 min after exposure to a calcium-free solution. It is concluded that calcium ions are important for the control of pacemaking, for the propagation of the impulse, and for the contractile response itself in these bovine lymph vessels.