Pacemaker frequency is increased by sodium nitroprusside in the guinea pig gastric antrum

Abstract

In the guinea pig gastric antrum, the effects of sodium nitroprusside (SNP), an NO donor, on pacemaker potentials were investigated in the presence of nifedipine. The pacemaker potentials consisted of primary and plateau components; SNP (> 1 microM) increased the frequency of occurrence of these pacemaker potentials, while inhibiting the plateau component. 1H-[1,2,4]-Oxadiazole [4,3-a] quinoxalin-1-one, an inhibitor of guanylate cyclase, had no effect on the excitatory actions of SNP on the frequency of pacemaker potentials. Other types of NO donor, (+/-)-S-nitroso-N-acetylpenicillamine, 3-morpholino-sydnonimine and 8-bromoguanosine 3'5'-cyclic monophosphate had no excitatory effect on pacemaker activity. Forskolin, an activator of adenylate cyclase, or 4,4'-diisothiocyano-stilbene-2,2'-disulphonic acid, an inhibitor of the Ca(2+)-activated Cl(-) channel, strongly attenuated the generation of pacemaker potentials, and SNP added in the presence of these chemicals restored the generation of pacemaker potentials. The pacemaker potentials evoked by SNP were abolished in low-Ca(2+) solution or by membrane depolarization with high-K(+) solution. The SNP-induced generation of pacemaker potentials was not prevented by cyclopiazonic acid, an inhibitor of internal Ca(2+)-ATPase, but was limited to a transient burst by iodoacetic acid, an inhibitor of glycolysis, carbonyl cyanide m-chlorophenyl-hydrazone, a mitochondrial protonophore, or 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid acetoxymethyl ester, an intracellular Ca(2+) chelator. These results suggest that the SNP-induced increase in the frequency of pacemaker potentials is related to the elevated intracellular Ca(2+) concentrations due to release from mitochondria, and these actions may be independent of the activation of guanylate cyclase.

Figure 1. Effects of sodium nitroprusside (SNP) on pacemaker potentials recorded from the guinea pig antrum

SNP (A, 1 μm; B, 10 μm; C, 100 μm) was applied as indicated by the bar under each record. D, high-speed trace of the pacemaker potential recorded in the absence (continuous line) and presence of 100 μm SNP (dotted line). The resting membrane potentials were: A, −63 mV; B, −62 mV; C and D, −64 mV. All responses were recorded from different tissues.

Figure 2. Effects of SNP on follower potentials and slow waves recorded from guinea pig antrum

SNP (100 μm) was applied to follower potentials (A) and slow waves (B), as indicated by the bar under each record. C, SNP (100 μm) was applied to slow waves in the presence of 10 μm ODQ for 30 min. The resting membrane potentials were: A, −60 mV; B, −67 mV; C, −65 mV. All traces were recorded from different tissues.

Figure 3. Effects of SNP on pacemaker potentials in the presence of forskolin

Pacemaker potentials recorded in response to 1 μm forskolin (A) and subsequent 100 μm SNP in addition to forskolin (B). C, D and E are responses of pacemaker potentials recorded during wash out of forskolin and SNP for 9 min, 24 min and 33 min, respectively. The resting membrane potential was −65 mV. All traces are continuous with some interruptions.

Figure 4. Effects of high [K⁺]_o solution on SNP-evoked pacemaker potentials in the presence of forskolin

A, SNP-evoked pacemaker potentials were recorded during application of 15.3 mm [K⁺]_o solution in the presence of 1 μm forskolin (applied as indicated by the bars under the record). High-speed traces of pacemaker potentials recorded before (B) and during application of 1 μm forskolin (C) and 1 μm forskolin with 100 μm SNP (D) are shown. All responses were recorded from the same cell with the resting membrane potential of −63 mV.

Figure 5. Effects of SNP on pacemaker potentials in the presence of 4,4′-diisothiocyano-stilbene-2,2′-disulphonic acid (DIDS)

A, pacemaker potentials were recorded before and during application of DIDS (300 μm) followed by 100 μm SNP (applied as indicated by the bar under the record). High-speed traces of pacemaker potentials recorded before (Ba) and during application of 300 μm DIDS (Bb) and 300 μm DIDS with 100 μm SNP at 0.7 min (Bc) and 3 min (Bd) are shown. Low [Ca²⁺]_o solution (C) and 15.3 mm [K⁺]_o solution (D) were applied to SNP-induced pacemaker potentials in the presence of 300 μm DIDS (applied as indicated by the bar under the record). The resting membrane potentials were: A and B, −64 mV; C, −62 mV; D, −67 mV. All traces were recorded from different tissues.

Figure 6. Effects of Ni²⁺ and Co²⁺ on pacemaker potentials

Ni²⁺ (10 μm) and Co²⁺ (10 μm) were applied to pacemaker potentials as indicated by the bar shown under records A and C, respectively. B and D are high-speed recordings of pacemaker potentials recorded in the absence (continuous lines) and presence of 10 μm Ni²⁺ or 10 μm Co²⁺ (dotted lines), respectively. The resting membrane potentials were: A and B, −62 mV; C and D, −62 mV. A and C were recorded from different tissues.

Figure 7. Effects of aminoglycosides on pacemaker potentials

Pacemaker potentials were recorded during application of 3 mm neomycin (A) or 3 mm gentamicin (B). C, high-speed recordings of pacemaker potentials, in the absence (continuous lines) and presence of 3 mm neomycin (dotted lines). The resting membrane potentials were: A and C, −64 mV; B, −61 mV. A and B were recorded from different tissues

Figure 8. Effects of Ni²⁺, Co²⁺ and neomycin on pacemaker potentials in the presence of forskolin

Pacemaker potentials in response to 10 μm Ni²⁺ (A), 10 μm Co²⁺ (B) or 3 mm neomycin (C) (applied at the bar under each record) were recorded in the presence of 1 μm forskolin. The resting membrane potentials were: A, −63 mV; B, −66 mV; C, −64 mV. All traces were recorded from different tissues.

Figure 9. Effects of SNP on pacemaker potentials in the presence of cyclopiazonic acid (CPA) or 1,2-bis(2-aminophenoxy)ethane-N,N,N‘,N‘-tetraacetic acid acetoxymethyl ester (BAPTA-AM)

Pacemaker potentials were recorded before (a) and during (b) application of 10 μm CPA for 30 min (A), 50 μm BAPTA-AM for 30 min (B) and 50 μm BAPTA-AM at 60 min (C). In the presence of these chemicals for 30 min (A and B) or 60 min (C), 100 μm SNP was applied at the bar under each record. The resting membrane potentials shown by dotted lines are: A, −63 mV; B, −60 mV; C, −66 mV. All traces were recorded from different tissues.

Figure 10. Effects of SNP on pacemaker potentials recorded in the presence of carbonyl cyanide m-chlorophenyl-hydrazone (CCCP) and iodoacetic acid (IAA)

Pacemaker potentials were recorded before and during application of 3 μm CCCP (A) or 1 mm IAA (B), and 100 μm SNP was added as indicated by the bar under the record. In B, 10 μm Co²⁺ was also added together with IAA and SNP. The resting membrane potentials were: A, −62 mV; B, −65 mV. A and B were recorded from different tissues.