Co-transmission and Blood Vessel Contractility

Nally, Jane Elizabeth (1990) Co-transmission and Blood Vessel Contractility. PhD thesis, University of Glasgow.

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Co-transmission in vascular smooth muscle was examined with a view to establishing the nature and mechanism of action, of each of the transmitter substances released in response to nerve stimulation. 1. The intracellularly-recorded electrical and simultaneous mechanical responses of the rabbit saphenous artery were examined in vitro in response to field stimulation of intramural sympathetic nerves and to exogenously applied drugs. 2. In the rabbit saphenous artery the resting membrane potential was -69.390.+/-26mV (n=250), the space constant (l) was 0.42 +/- 0.12mm (n=12). Field stimulation evoked excitatory junction potentials (EJPs), which facilitated (>1Hz) and summated (>2Hz). These EJPs were abolished by infusion of alpha,beta-methylene ATP (alphabeta MeATP 10e-6M), but were unaffected by prazosin (10 -6M), suggesting that they were mediated by an ATP-like substance. There was no additional electrical event which was associated with NA, implying that NA does not evoke a significant voltage-dependent response in this artery and may act via a voltage-independent mechanism. 3. Exogenous application of ATP (10 -2M) or its stable analogue alphabeta MeATP (10 -4M) by close pressure ejection from a micro-pipette, produced dose dependent depolarisations. NA (10 -2M) similarly applied was ineffective. Theses results are consistent with the view that the transmitter substances, NA and ATP (or a closely related substance), released from the rabbit saphenous artery produce dissimilar electrical effects. 4. Action potentials were not observed in the absence of drugs in the rabbit saphenous artery. In the presence of tetraethylammonium (TEA, 5x10 -4 -10 -3M) however, action potentials were observed in response to both single stimuli and trains of pulses. This suggested that action potentials are normally absent in this tissue as a result of a fast rectifying K+ -current, but can be sustained when this conductance is inhibited. 5. Alteration of the external ionic environment by changing [K+]o, [Na+]o and [Cl-]o suggested that changes in K+ and Na+ conductance may underlie the EJP produced by nerve-released ATP. 6. In the rabbit saphenous artery, approximately 50% of the contraction was abolished by alphabeta MeATP (10 -6M), while the remainder was abolished by prazosin (10 -6M). Both NA and ATP therefore contribute to the mechanical event, supporting their co-transmitter role in this tissue. 7. All electrical and mechanical responses were abolished by either tetrodotoxin (TTX, 10 -6M) or guanethidine (10 -6M), indicating that they were due to transmitters released from the same sympathetic nerves. Idazoxan (10 -6M) enhanced both the electrical and mechanical responses, suggesting that a2-adrenoceptors can pre-junctionally modulate release of both transmitters. This supports the view that both ATP and NA are released from the same nerves. 8. The P2x-purinoceptor desensitising agent alphabeta MeATP was shown to be a suitable selective compound for analysis of the ATP component of co-transmission in the rabbit saphenous artery. No evidence was obtained from examination of the effect of alphabeta MeATP (10 -7-4x10 -6M) on membrane conductance by the method of Abe & Tomita (1968) for a non-selective membrane effect of this compound, nor were any differences between the effect of alphabeta MeATP on the evoked EJPs and on the response to exogenously added ATP observed. alphabeta MeATP did not alter the contractions evoked by NA(10 -7 -10 -5M), by histamine (10 -6 -10 -4M) or by the post-junctional actions of KCl (1-1. 6x10 -2M). 9. Suramin abolished evoked EJPs and attenuated the contraction evoked by ATP (10 -5-10-3M) and alphabeta MeATP (10 -7-10 -5M) without affecting the contraction to NA (10 -7-10-5M) or the post-junctionally evoked contractions to KCl (10 -2-1. 6x10-1M). It however attenuated the contractions to histamine (10 -6-10-4M) and 5-hydroxytryptamine (5-HT, 10-6-10-4M) and thus seemed less selective as an antagonist of P2-Purinoceptors in the rabbit saphenous artery. 10. NA failed, in the rabbit saphenous artery, to evoke hydrolysis of phosphatidylinositol 4,5-bisphosphate (PIP2), as measured by incorporation of myo- [2-3H] -inositol into the water soluble metabolites of PIP2 hydrolysis - the inositol phosphates. This was the case even in the presence of propranolol (3x10-6M) to inhibit any inhibitory effect of beta-adrenoceptors on PIP2 hydrolysis. In the rat tail however, used as a positive control, NA produced a dose dependent hydrolysis of PIP2. This implies that PIP2 hydrolysis is not the basis for the voltage-independent mechanism by which NA produces contraction in the rabbit saphenous artery.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Medicine
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78183
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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